Molecular Structure Studies on the Infrared

Citation preview

MOLECULAR STRUCTURE STUDIES II THE INFRARED

by Henry Tice Hoffman Jr®

A dissertation submitted In partial fulfillment of the requirements for the degree of Doctor of Philosophy* In the Department of Chemistry in the Graduate College of the State University of Iowa August 1950

ProQuest Number: 10666172

All rights reserved INFORMATION TO ALL USERS The quality o f this reproduction is d e p e n d e n t upon th e quality o f th e c o p y subm itted. In th e unlikely e v e n t th at th e author did not sen d a c o m p le te manuscript an d there are missing p a g e s , th e s e will b e n o ted . Also, if material h a d to b e rem oved , a n o te will in d icate th e deletion .

uest, ProQuest 10666172 Published by ProQuest LLC (2017). Copyright o f th e Dissertation is held by th e Author. All rights reserved. This work is p ro tected again st unauthorized cop yin g under Title 17, United States C o d e Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346

V\to^A-

CL.O V\

ACKIOWLSBGEM^ TS The writer wishes to express his gratitude and thanks to all members of the faculty and staff, and fellow students who have helped directly or indirectly with the work of this research. Special acknowledgements are due to the followings To Dr* George Evans who conceived the problem studied, for willing assistance and guidance at all times* His aid in the inaugural of the experimental work and eval­ uation of the data is very much appreciated* To Dr* George Glockler who assumed the burden of this research project when Dr. Evans left, for valuable time spent in discussion and for helpful suggestions and assistance in carrying the project to conclusion* To Mr* Wade Hall, for help and cooperation in the operation of experimental details. To Mr* Harry Bunn&aaker who constructed and helped design the special absorption cells for this project* To the Allied Chemical and Dye Corporation for a research fellowship grant under which a portion of this research was done.

f o) jL u

It

FOREWORD Ethyleniraine and N-raethyl ethylenimine are repre­ sentative members of the three mambered ring system of compounds* properties

A comparison of their physical structural other members of this series, name iy

cyclopropane and ©thylen© oxide, shows marked similarity, particularly in the case of ethyleniraine. I'he fundamental frequencies of like groups in these molecules should be of the same order of magnitude* The assignments of the fundamental vibrational frequencies in the case of cyclopropane and ethylene oxide have been made by previous investigators through various studies of their infrared and Raman spectra*

Evaluation and cor­

relation of these data is given by Herssberg (16, p* 541, 351 ff*). In the present study, an assignment of the funda­ mental vibrational frequencies is given for ethylenimine through a study of its infrared spectrum and consideration of the Raman spectrum and polarisation measurements of Kohlrauseh and Reits (21).

A comparison of assigned

frequencies with the accepted values for cyclopropane and ethylene oxide is given*

A heat capacity calculation Is

also made for ©thylenimlne* ill

Th© Infrared spectrum of H-methyl ethylenimine is compared with that of ethylenimine, as the occurrence of any significant differences can be ascribed to the effect of substituting a methyl group for th© imine hydrogen#

iv

f w i of 001?w m dhapber 1

Fftg©

Introduction* • » * . * • . • * « • * » • * « » • Historical Background* * • ........... • * Statement of the Problem . . . . . . . . . .

If

Instrumentation and Methods . . . . . . . . . . . Infrared Spectrometer* * * # « * * * . * * * Performance feats*............. Boise Bowl* thermocouple tests, Besot* lution ©hecks. Slit Reproducibility test, Brum Settings, feats, for Scat­ tered Bight, Slit Performance Testa, Amplifier tests, Drift tests* « * * . * Operational Procedure* # « * , * * • » * * • Calibration of Prisms * • , * * • * • • « * * Soils and Apparatus. « * * * « * * • * • • * Obtaining Percent Transmission Data* * * * *

III

Preparation and Purification of Ethylenimine and B-Methyl Sthyleniwin®* » * # * * * * • *

2. 1 5 0 S 11.

11 @0 14 SO 36 43

Itbylonlmim# « , . * * * * . * * • * * * # * • 43 M-Methyl Ethylenimine*..................... 47 It

V

Theoretical Discussion. ........ Vibrational Spectra. * * » * . * » » . * » . Rotational Spectra • « • • « * * * • . * * *

51 58

Experimental Results and Discussion » « * « * * «

64

Ethylenimine * * * • * « « * • « • » . * * # Hydrogen Bonding* • « * * • • * • * * « Jtssigns^nt of Fre«|uen©ies » * * « . * • HoatCapacity Calculations. . . . . . . B-Methyl Ethylenimine. . * , * ............. Assignment of Frequencies • « » , « • • VI

51

Summary • * * * * » • * • * * * • * • • « « * • • Bibliography*

64 7f 7® 85 87 88 99 101

TABLE OF FIGOBES

?%w**,

1{ ’2

1

, ,

J Slit ! ? i Performance

*»#•

Test. . . . . . . . . . . .

13

‘Ipood w M 1SbaHftP Sotacdulo (M&01 Fniam)* * # 24

‘

4-

ttoaQvpttm Colls# # . ♦ * * •• § « « 14 OalX Filling Apparatus ’* • » » « » • «

« •■ # IS

B

'Ifedol of Mttsyloninin©*' « * * * # » » * « * * 04

4

„

7

,Xnfr&tfSd B p m t r m of Bthylontzaino 6a$* * * » 67

v8.

,$®pf«so&tfttion of SXsmontoory Modoc of Motion ,of Spooffio Owopo * * ,# * « * « » » * * » * 74

,9.

Band lavolopoo for Et&ylonlialn©# * 61

Xafrcrod Speefcrum of M~Mothyl EtBylonimln® 0fto * * * « » # «

»

Vi

# ♦ * * . * * * # * * *

69

fABSUK m

TABLES

Table

F*ge

I

Thermocouple Sensitivity* * .#* « « » * • * « •

'tl

Comparison of Experimental Frequency Baines'of Liquid Benaene infrared Bands # * # # * * . * * *

XXX

leprodmeiMilty of intensity Measurements * • *

41

XV

Comparison of Transmission of Cell A and Cell B

42

f

Symmetry Species for feint Croup 0 • » « • . *

55

Humber of Vibrations of Bash Species for feint Croup C0f # * * # # * * * * * * * . * * ♦ * # # *

id

n

VII VIII

14

Species of fundamental Vibrations m d Activity ........ . • , . 52 for e^Hg, CgH^Of and C^H^HE Slit Schedule for Ethylenimine Cat* # * * # * •

65

IX

Slit Schedule for blquid Ethylenimine * * * • »

66

X

The Observed Frequencies of Infrared Bands of Ethylenimine With .Estimated Intensities * * * *

70

Comparison of Observed Frequencies for EthylenImlne In the 3 micron region* * * « • » • • « .

73

XX XII XXXX XIV XV XVI

Symmetry- of lormal 'Modes of Vibration of Ethyl** m i m i n e ............................. ( • 7@ Fundamental Vibrational Frequency Assignment® for Ethylenimine * * * * • * * • » « « . » # * • #

30

Assignment of Combinations and Overtones for Ethylenimine in th© Infrared# • • « « « • * • «

81

Comparison of Assignment of Fundamental Fre­ quencies t C3H6# CgH40, and C g H ^ H ............ Calculation of C * for Ithylenlmin© 8&® at 371 #23°K . # * Jirt # * # # # * * * # • * # # * vil

82 86

TABLE rn TABLES, (continued) Table

mu 3CVXXI

Page Slit Schedule for X-M@thyl tfchyXenimfne Gaa ♦ « SB fhe Observed frequencies of Infrared Sand* of-. M-M@tbyX Ithylenlmlne ©as with Estimated inten** titles * * * « * 4 * . » » » * » • « * « * # » ««

SS

XIX

Symmetry of Hormal Medea of Vibration of SMfatfcfl Ethylenimine* * # » « » * * « * « # * * * « * » 94 * *

XX

Partial Assignment of fundamental Frequencies for X*M@thyl IthyXenimine * * . f * # * » » * « . « # 95 .,

■.

* i *

XXX Suggested Overtones and Combinations for I-Methyl Ethylenimine* . * # • * * * * * * . . * * • . * 96 XXXX

Comparison of Fundamental Frequency Assignments of Ithylenlmlne and [email protected] Ethylenimine ,* 6 ;*,*

viii

9®

1

Chapter 1 IOTBOD0CTIOT Blatopical Background Th© infrared portion of the ©1eetromagnet 1c spectrum lies between the red end of the visible region and the ultra short radio waves*

It is divided into th©

following approximate divisional

The photographic region

from 0*75/f to X»3jjl* the near infrared region comprising th© overtone and fundamental vibrational regions from 1*3/1 to 2#5jjL and 2*£j/jLto

respectively* and the far

Infrared consisting of the rotational region from 25JAto 350a*

Units usually used to measure infrared raclljA

ation are the micron (ja9 » 10 the wav© number

cm*)* wave length unit and

a frequency unit*

The discovery of infrared radiation was made by William Herschel in 1800 while making temperature measurements of different parts of the sunfs spectrum* He observed that a temperature rise occurred throughout th© visible rang© with respect to a reference thermometer, but also found that the highest temperature rise occurred in a range just beyond the visible rang© of the r©d portion of the spectrum.

By contrast he found that a

visual Intensity maximum of the light occurred in the

2

yellow-green portion, and therefore postulated that two different types of radiation were involved, a heat radi­ ation and a light radiation*

It remained for later in­

vestigators to show that these two types of radiation are similar In nature, differing only in their degree of refraction by a prism* The lack of suitable detecting devices for the weak radiation of the infrared retarded th© investigation of this part of the spectrum*

Th© discovery of the ther­

mocouple, sad subsequent development of the thermopile, bolometer, and other sensitive detectors put the investigation of infrared radiation on a sound experi­ mental basis*

With these detectors it was possible to

study much narrower frequency ranges*

The use of a

bolometer in connection with a sensitive galvonometer enabled bangry to use a grating for accurate wave length measurements • Basically m o dem infrared instrumentation consists of a source of continuous radiation in the desired spectral range, a monochroma tor, and detector-amplifier-recorder system* The Ideal source is a blaekbody radiator which gives continuous radiation In the desired region*

There

are several sources which approximate this condition, the

3

Grlcb&r and $emst glower being th© most widely used. In th® near infrared, a monochromator, usually prism type for general work is used in dispersing and collimating desired frequencies on the detector*

Mirror

optics are employed in either a hitfcrow or Wadsworth mounting*

Selection of prism materials depends on th®

spectral range being studied* fh® detector is usually a thermocouple or bolometer connected with an electronic amplifier and recorder*

Th© recent development of fast, sensitive

detectors and stable amplifiers has made possible co»«* tinuous scanning and recording of a given spectral range* It was early discovered in the study of infrared absorption of organic molecules that a definite relationship existed between the characteristic absorption frequencies and th© structure of the molecule*

The work

of Ooblenfcs and others in the studies of molecules containing similar groupings showed that a particular group would cause absorption within a definite frequency rang® quit© irrespective of the rest of the molecule* By considering a molecule as a mechanical model of point masses governed by Hook’s law forces, BJerrum was able to calculate some of the fundamental frequencies* Comparison of the calculated with experimental infrared

4

and Eaamn data h&a permit tad fundamental frequency and force constant assignment for most of the simple molecules* Values for these data combined with those of moments of Inertia allows a calculation of specific heats and thermodynamic constants# A detailed review of th© historical aspects of Infrared spectroscopy Is given by Earner and Bonner (3) and (4)*

Barnes, at# al# (5)| Williams (54}| and Harrison,

et# al# (15) give recent summaries of the field with emphasised* Instrumentation and applications*

Treatment of

the theoretical aspects with particular relation to molecular structure and an evaluation of most of the work done In this field up to 1943 Is given by Eerzherg (15)# Other theoretical treatments are given by Sutherland (29)j Bawllss and Taylor (27)j and Schaefer and Matossi (28)* Annual reports of recent developments and investigations appear In Analytical Chemistry (14)#

5

Statement of the Problem On the basis of Its chemical reactions (24), ethylenimine appears to be a three membered heterocyclic ring similar in structure to ethylene oxide and cyclo­ propane*

Th© latter two molecules are conceded to be

equilateral triangles (16, p* 341, 351 ff*)«

It may b©

assumed that ethylenimine Is also nearly an equilateral triangle. The structural similarity can be shown by a consideration of the physical characteristics of the methylene, oxygen, and imino groups*

The masses of

these groups are very close, 14, 16, and 15 atomic weight units for the methylene, oxygen, and Imino groups respectively*

These groups are also iso-electronic*

It

is therefore to be expected that the force constants of these molecules are of the same order of magnitude and one may expect similar frequency values for groups characteristic to all three molecules*

However, the

observed spectra would be quite different in selection riles due to the different symmetry of the molecules# (See Chapter IV). The Raman spectrum of ethylenimine has been observed by Kohlrausoh and Kelts (21)#

They also gave

polarization data but made no assignment of frequencies.

6

%»t©r (10} studied ifchylenimln© In th© photographic infrarod and observed wall defined band© of an assjrmetrle rotator* In the present work the Infrared spectrum of ethyl©Biinin© was studied from 4000 era1 *1 to 667 cm*1 using Lit and laCl priams*

studies were mad© In th© liquid as

well as th© gaseous state in order to obtain better correlation with the K&man data* to assignment of the fundamental vibrational frequencies was mad© on the basis of these data obtained from Infrared studies and a consideration of th© following factorsi laman spectrum and polarisation data {01}* in­ frared band shape where it Is resolvable* approximate intensities* distinguishing fundamental® from overtones* characteristic group frequencies* and correlation of assignments given for ethylene oxide and cyclopropane* Using these fundamental frequencies and assuming that translational and rotational contributions are classleal, th© heat capacity at 871*03®& was calculated*

fhis

was don© to compare th© value obtained with th© calculated heat capacity of ethylene oxide at th© sam© temperature (13)* The infrared spectrum of H-methyl.ethylenimine was studied through the same region and a partial assignment of fundamental frequencies based on similar

7

arguments was made*

s Chapter II IWTIHJM2OTATI0H AMD METHODS Infrared Spectrometer ■wSSSSSSESHS^ iir^^ujmwi^Sii>ia Because molecular structure studies by means of infrared spectroscopy have just recently begun In this laboratory, Instrumental methods and techniques in use here will be considered in soma detail# Infrared spectra were obtained with a PerkinSlmer, model 12 ~C infrared spectrometer#

This instrument

is designed for AG operation and is known as a modulated beam type spectrometer#

This method of operation Is said

to be effective in the elimination of thermocouple drift which has always been one of the major difficulties In infrared instrumentation#

Conventional DC operation i®

also possible# Th® monochromator Is an Interchangeable prism type using a Littrow mount#

Chrome-aluminum mirror optics

are used for high reflectivity#

The desired frequency

can be brought to focus on the exit slit of the spectro­ meter by rotation of the Littrow mirror#

The mirror Is con­

trolled by a wave length, drum which is a micrometer divided Into arbitrary scale units#

The wave length drum

can be set manually or can be operated by an automatic

9 drive for continuous scanning of a given range*

The wave

length drive Is geared to a synchronous motor and a selection of four speeds Is available for transversal of the spectrum* The source of radiation is a commercial Globar* It is housed in a water-cooled jacket* the Globar Is regulated by a Vari&c®

The voltage across Kadiation from the

Globar is focused on the slit entrance of the monochro­ mator with mirrors contained in a source housing* Hoorn for placing the sample in the optical path of the radiation is provided between the source housing and the entrance slit of the monochromator*

Three shutters,

opaque, LiF, and glass are fitted on the source housing so that th© radiation from th© source can be cut off without closing the slits* A chopper assembly placed directly in front of the Globar Interrupts the radiation at approximately seven cycles per second*

'^fais produces an alternating voltage

at the thermocouple and by suitable filtering after amplification, the signal can b© separated from DC voltages caused by ambient temperature changes taking plciCv at the thermocouple*

In this manner thermocouple

drift Is reduced* The detector is a pin type thermocouple*

It

10

is a permanent vacuum type and Is fitted with a KBr window* Characteristics are its high sensitivity and fast response. Output from the thermocouple is amplified by a breaker type electronic amplifier*

The output from the

amplifier is fed through a filter to separate the true signal of th© source from the DC voltages due to temper™* ature effects. Finally th© signal is fed to a recorder which is a pen type Brown strip chart recorder*

Fiduciary

markings are produced on the chart by an impulse of th© pen at every tenth division of the wave length drum as the speetium is scanned.

The pen is aotivitated by a

wav© length marker so coupled with the wave length drive to cause a discharge through the recorder at every tenth division*

Frequency values are obtained by use of cal­

ibration curves which are plots of wave length versus frequency.

(See under calibration). A detailed description of the spectrometer and

its operation is given in the instruction manual (18), hereafter referred to as "the manual".

11

Performance gTests aaerawB Performance tests on th© component parts of the spectrometer were made In accordance with instructions In th© manual®

^'hese tests will be discussed in some

detail as they give insight on th© operating character­ istics of the machine and the conditions under which the infrared studies were carried out*,

These tests must be

mad© at-frequent intervals as th© machine does not maintain its level of performance indef5nit©1y* Boise Level® Boise in the circuit 1® indicated by the rand cm peak to peak variation of the recorder pen when the spectrometer is in operation®

The noise level

was found to be acceptable under the conditions of the tests at times, but was not consistently so®

It was

usually found helpful after allowing the spectrometer to warn up for as long as two or three hours* to turn all the controls on as In actual operation and run for about fifteen minutes.®

During this time the noise level would

usually decrease from its level at the beginning® The noise level in tems of muV. (microvolts} was determined by comparing its width with the deflection of the pen produced when a signal of known value was sent through th© recorder*

^he amplifier is fitted with, a

switch calibrated in ®1, 1, 10, and 100 muV• The switch

12

whan set to on© of thes© values causes 8 discharge through, th© recorder producing a deflection or the pen proportional to the signal*

The magnitude of the deflection of any

given signal depends on th® amplifier gain used.

By this

means the voltage of any signal sent through the recorder can be determined by comparison of th® deflection that it produces with that of one of the known signals.

Ibis Is

the method used in all tests where results are ©3spreseed in terms of muV.

The magnitude of the deflections in

these tests is measured by finding the aero point (pen position on chart with shutter in or signal off).

A

signal is sent through the recorder, either from the source or the amplifier causing a change in pen position,

‘ ^his

change is known as the deflection and is usually measured in terns of chart units or in centimeters. Thermoooupie tests i These tests were made on a new thermocouple which was installed after the old thermocouple in use was broken.

Since no output could be

detected from the old themocouple after certain inlet pipes had been installed on the base of the spectrometer, it was assumed that the thermocouple junctions had been broken by vibration.

Therefore it mu at be emphasised

that------— utmost must be the -the m■.o.-. --- care —■ -.t--r i--i-mi. ii r —i -,taken 1- irr-rr.jrJ in ^ handling ~~ ... -----

13

couple or performing any operation which may possibly disturb it or Jar it. The 'new tbeBnocoupl© was rated by th© PerkinElmer Corporation as follows?

Serial no* 354 j rated

sensitivity, 5*4 mu¥/muW* and resistance 11 ohms# It was necessary to refocus th© elliptical mlxrror after installation of the thermocouple*

The

elliptical mirror possesses three adjustments for focussing, horizontal and vertical adjustments, and a center adjustment for forward and backward motion. Position of best focus was determined by making adjust— rsents until maximum deflection of the pen occurred*

The

adjustments were carried out in the following manner* The horizontal adjustment was altered till maximum deflection of th© pen was found, the machine being in operations

Center and vertical adjustments taken in this

order were made in the same manner*

The center adjustment

was altered after each vertical or horizontal adjustment. In this manner it was quite easy to reach th© point of maximum deflection where alteration of any of the three adjustments would not increase the deflection,, Thermocouple tests were made with settings as mentioned below*

It may be noted that the slit-zero was

taken as .025'mm., this position. bein.:, the point at which

14

deflection of the pen was observed when the slits were opened slowly at the given control settings.

However, the

tero point as determined by the slit performance test (see under slit performance) was ,010 mm.

The Globar was

operated at 50 volts, Which is approximately th® correct voltage for a power output of 200 watts.

Following are

the spectrometer settings used (with the faOl prism)* Controls

Short lave Length

Variac Brum Reading Hilt sere Slit Response

Long Wave Length

SO volts 30,45 ,033 mm, ,123 mm,

50 volts 6,9?

,023 mm, 1,023 t o ,

2

2

Results of the test are tabulated in table X, Table X Thermocouple Sensitivity

Operation

Wave Length

Deflection**

Short

15

Long

30

Short Long

45

e In ohart units.

a

5,5 70 14

Output 55 imxV. 1,4 87 3*1

15

According to the manual the output at short wave lengths should be sixty tebuV* Be and 45 muV, AC*

The

high output of eighty muV* observed on DC operation can be attributed to using too high a setting for the slit~&©ro or wanning th© source at more than th© stated 200 watts* The low output of 51 muV* on AC was probably due to misphasing of the chopper assembly* It was fotmd however that the new thermocouple did not maintain the output found In the above tests* After operating over a period of several months, th© output was lbunci to have dropped to 13*5 rauV* AO and 27

DC*

From all indications. It sesmed that the

drop of output was due to a drop in sensitivity in the thermocouple since performance of other components remained on a par with previous tests* Resolution checkst Satisfactory resolution checks were obtained on the 4*2

Carbon dioxide band

with the $&€! prism using the following settings* Variac, 50 volts Response, 2 Speeds, 4 & B Bain, 6*9 AC operation Atmospheric water vapor bands were also resolved, satis­ factorily in the $aCl region using the recommended

16

settings*

To obtain satisfactory checks for the 3

water

vapor bands using the IdF prism, it was necessary to use a *05$ mm* silt rather than the specified *073 ram*

It may

be noted that these settings were the actual slit-set tings without th© slit zero values being added*

Slit reproducibility'test* Maximum deviation frcsn the average of three tests was of the order of three per cent compared with the maximum of one per cent stated in the manual* Drum settings * Reproducibility of a given drum setting for a particular band was satisfactory* Tests for scattered light* & test for scattered light was mad® at 14*5

in the &&C1 region*

corresponds to a drum setting of 7*42)*

{This position

For a total

deflection of 35*5 chart units, the LiF shutter gave a deflection of 3*5 units*

It gives a value of th© order of

ten per cent for scattered light and falls within the prescribed limits* Slit performance tests * The entrance and exit slits of the monochromator are bilateral slits*

They

are controlled simultaneously by the slit micrometer which reads directly In thousandths of a millimeter from 0 to 2 mm.

For th© slit performance test, the wave length drum

was set at the peak of th© energy curve {the frequency where the greatest amount of energy Is radiated by the

17

globar) and the deflection produced by a given slit setting was measured*

When the square root of deflection was

plotted against slit width, the value for the slit-zero was found to be *100 mm*

^h© following spectrometer settings

were used with the M a d prism*

Variac, 50 volts Response 2 Brum 20*45 Gain 5 The deflections observed for given slit widths are tabulated below and a plot of the square root of deflection versus the slit width is shown in figure Is

Slit width (am,} *050 *040 .045

Deflection (Chart units) 21 ,5 48.5 66.0

(Deflection)c 4.63 6.96 8.12

Amplifier testss Th© amplifier in addition to the microvolt-test-signal-switch has the following additional controls*

The degree of amplification is regulated by the

gain control which can b© set at arbitrary units from 0 to 10*

Balance controls, one regular, and one for fine

adjustment permit placement of the pen on th© chart at any

18

10

9

a H|Gi

I? O o

•H -P

i 6

O 0

(H 0

5

o

o 4 -p o o

(«

3

0

h

cd ft CJ1 cn

0

|0

20

30

40

SO

60

Slit Width (mm. x 10*^) Pig, 1

Slit Performance Test

19

any desired points

For operation, the off~on switch and

battery switch must b© turned on*

Tho amplifier is

provided with a shorting cap at the side which permits testing of the amplifier without the thermocouple being in the circuit* Output *

{Shorting cap in circuit}*

At full

gain *1 muV* causes a deflection of 51 units on AC and 68 units 00*

For a given test signal the output on AC has

been found to vary by as many as 12 units during the lapse of several days* Molso Level* WW wiwiwwwiii C hmmpmmmmm*

Hois© level ranged from *002 muV.

to *004 rauV. on AC* Linearity* Maximum deviation amounted to 2 units which is within the prescribed limits*

Gain setting was

about 5*8 and 1 muV* gave a deflection of 33 units* Drift» Tendency to drift was not as bad as when themocoupl© was In the circuit* The test battery used in the amplifier had a voltage of 1*57 volts*

The performance of the amplifier

was checked against that of a loaned amplifier and its perfoimanc© was as good as the behavior of the loaned instrument* Drift tests *

(With the thamoeoupl© in circuit)*

One of the greatest difficulties encountered in the oper­

20

ation was that of drift, i. a., shift of the zero point. When the spectrometer is in operation and no recording of spectra is being done, th© pen should produce a straight line.

When the spectrometer was operating at its

best, a shift of *5 to 1.5 chart units was noticed during the course of IS or 20 minutes.

Buns were made under these

conditions as far as possible as knowledge of the drift is essential in obtaining data for per cent transmission curves. Adjustment of th© balancing control on th© back of the filter box did not lessen the drift.

At times it

was not possible to bring th© pen to th© sero mark by this ad justment» Operational Procedure

■ m ih i l K w i . i n i i m i i

i i n w i ii

ihp i - in

i w M i an w i j

The operation of other controls not discussed previously in connection with performance tests will be considered her©. The filter box in addition to the aero balance control has a switch for AO or DC operation and a response control.

The response control regulates the speed of

response of th© overall system.

Speed of response is

decreased in going from position 1 to 4.

Position 2 is

recommended for normal operating conditions and was used

21

throughout these studies, The motor control knob located to the left of the wave length drum controls th© wave length drive.

It

©an be left off for point by point transmission measure­ ments or when Bonw provides continuous scanning of a spectral range,

Th© speed control Is to the right of the

motor control and provides speeds of 1, 2, 4 or 8 minutes per drum revolution, Th© control panel contains a switch for power supply and start«stop switches for placing the spectrometer components into operation for recording of the spectrum. Before actual operation, th© machine must be allowed to warm up for a period of time.

As stated before

two or three hours were necessary for best operation.

The

first step in starting the machine is to make sure that water Is running through the globar jacket,

To warn up

th© machine, th© power supply switch, and both switches on th© amplifier ar© turned on,

Controls for which the

settings are usually constant are th© variae voltage which Is usually operated at §0 volts and the response control which Is sat at position 2, Th© gain control on the amplifier was set as high as possible consistent with stable operation.

This

was done to use narrow slits In order to obtain maximum

resolution.

It was found that a gain of about 7 was the

maximum that could b© used consistently.

Since the

output of the amplifier varied somewhat from day to day, th© gain was set to give a deflection of about 12 chart units per ,1 mu¥.

This condition represents a full seal©

deflection for approximately *8 rauV,

It Is approximately

the degree of amplification used by other investigators on this type machine (26) ® Th© gain setting required to give this deflection was usually 8,9 to 7,2,

Slits were

set at a value to give 80 or 90 units at the beginning of a spectral section. When scanning through the spectral rang© of a given prism, the only control changes necessary are th© slit settings,1 motor speed, and shutters® Since th© energy of the source decreases quit© rapidly as a spectral rang© Is transversed,, the slits must be opened frequently during the run in order to maintain a high deflection for the background (deflection due to th© radiation of the source with no sample In path). In these studies a fixed slit method was used* A slit was selected that gave a deflection of 80 to 90 units for th© background at the start of the run®

The run

was allowed to progress to the point where the background dropped to about half®

The run was interrupted at this

23

point and slits were opened to give a high deflection* fhis spectrometer has a continuous silt drive attachment which permits scanning of the entire range of the liaCl prism in one operation*

It was found that In

use, it was difficult to obtain reproduclbllity of slit reading with respect to wave length drum reading*

fhis

reproducibility is desirable in obtaining data suitable for per cent transmission curves« However, the continuous slit drive is convenient for preliminary runs as a quick method in locating position of bands* In this work the selection of scanning speeds was made in order to obtain th© highest resolution*

Sine©

the dispersion of a prism becomes greater at the longer wav© lengths, it is possible to rim at faster speeds as th© spectrum is transversed from th© short to the long wave lengths*

For the NaCl region, the following speeds

were recommended in the manual, speed 8 from 2*5yu to 3.5^u, speed 4 from 3*5yjL to Q j x 9 speed 2 from §JJ to lOyULand speed 1 beyond* In order to reduce the effects of stray light, th© spectrometer is provided with three shutters to be used in different spectral ranges for determining the zero point* Shutters recommended for the various portions of the spectrum are as follows: opaque from 2.5uto 4*"* ,

24

glass fm m

to

I#iF beyond 9*0^u#

Figure 2 shows the range of speeds and shutters in terms of wave length drum readings which wei’e used for runs in the laOl region*

fhe calibration correction

factor (see under calibration) was considered in determin­ ing these settings*

When*using th© LiF prism, the opaque

shutter was used throughout th© entire rang©*

Speed 4 was

used to about 9*00 on the wave length drum and speed 2 beyond* LIFshutter

^ ^ Drum

Opaque ^ ^ Class shuttea^ ^hutte^

Speed 2

y ppe®d ^ ^peed ^

t 6.53

20745"“ Fig* 2 Speed and Shutter Schedule (Bad prism) For step by step procedure used in putting the

spectrometer into operation see pp* 27*29 in the manual* Calibration of Prisma The wave length drum of the spectrometer is a micrometer divided into arbitrary scale units numbered from 0 to 20 and can be read directly to *01 units * As

26

mentioned previously, this drum is attached to a wave length marker which causes a small deflection of the pen

at ©very tenth division aa the drum is rotated during spectral acanning*

These deflections are usually called

fiduciary markings,

Th© fiduciary reading of any point

on th® absorption curve is converted to wav® lengths with the aid of calibration curves. Data for calibration curves were obtained by running the spectrum of compounds whose absorption band wav© lengths are known accurately*

A sufficient number

of compounds were run to cover the spectral rang© of a prism.

Wav® length drum settings were found for

absorption peaks whose frequencies ar© known,

A plot of

wav© lengths versus th® corresponding drum setting gave th© calibration curve. The drum reading values of absorption peaks which did not fall on one of th© fiduciary marking was determined by measuring the distance between two

successive markings with a millimeter scale*

The chart

drive of th© recorder is set so that at speed 4 there are 10 mm, between markings, each mm, corresponding to *01 unit on th© drum scale,

At speed 2, th© distance between

markings is 6 millimeters and each mm* is ,02 units. The &aCl prism was calibrated by Dr. George

26

Evans*

Th© compounds used for calibration were atmos­

pheric water vapor and carbon dioxide bands, hydrogen chloridej, ammonia and ethylene oxide* was calibrated by the writer*

The LiF prism

Ammonia, hydrogen chloride,

hydrogen bromide, water vapor and carbon dioxide bands were used for th© calibration of this prism*

Identifi­

cation of the bands with respect to wave lengths was %uite simple with the aid of reproductions in the manual* -

In day to day operation, there was sometimes

found to be a calibration shift, i*©,, the drum reading of a given wave length had changed*

Whether such a shift

had taken place or not was easily observed by noting the positions of atmospheric bands*

It was made a point to

make a check during each run and applying an appropriate calibration factor which proved to be linear throughout the prism range* When runs were being made with the NaCl prism, the calibration was checked by noting the positions of the 2367 sharp 3760 em

and 667 cm**3, carbon dioxide bands*

The

absorption peak of atmospheric water and

2342 eaa'*^ band center of carbon dioxide were used for checking the LiF prism*

Since these bands show up In

©very run through these regions, an internal calibration check was always present*

27 Th© calibration correction factor for th© KaCl prism was usually of the ox*&©r of D or

Spectrum

e

o, >N

of Ethylen inline Gas

67

8 0 1 4 19 94 1 11 80 01 4 71 46 0 a 1 8 4 8 > 1 8 0 « 1 8 8 5 1 0 0 7 4 8 1 8 8'x818 * 1636 ■t

Thera la found a weak band at S36B cm"! which la suggest lire of a free 11 stretching frequency.

This

could be attributed to an impurity or can be assigned as an overtone# The total expected number of CH valency vibra~ tions in the 3000 cm*"** rang© are seven*

Six are found in

fefa. range 3X99 oar* to 8793 cm”1 and these are assigned as OH stretching vibrations* As in ethylenlmine and ethylene oxide, there are found three strong frequencies in the corresponding range of values which are assigned as the three ring deformation vibrations#

These frequencies are 1201 em*^, 818 cm" »

97

and 740 Four CHg deformati on vibrations are expected In th® 1450 ob*^ range*

Actually only two frequencies are

found in this rang©*

Theyar© 1466

and 1444 m ~ l

and ar© assigned as th© 0 fi^ deformation vibrations* In the infrared spectrum of methyl amine, there appears a band with a strong central maximum {% branch) centering at 1045 ©sT* (8 ) which is assigned as the 0