William A. Guillory Howard University Washington, D. C. 20001
The Infrared Spectra of Gases at 3 0 0 " ~
I
and of Frozen Films at 77°K
I
A physical chemistry experiment
D u e to the updating of the general and inorganic chemistry laboratory courses, many of the experiments formerly done in physical chemistry are now being performed in these earlier courses. I n addition, the theoretical nature of some physical chemistry experiments should be altered such that they differ significantly from those performed in instrumental analysis. Finally, experiments in physical chemistry should address themselves to the techniques and operations which are directly applicable to research being done in the field today. This experiment in infrared spectroscopy attempts to address these changes and incorporate these features. The objectives of the experiment are the following a) acqusint the student with the interpretation of infrared spectra, particularly as the absorptions relate to vibrational motions h) demonstrate the implications that can be derived from the various symmetry operations and point, group designations (Principle of Mutual Exclusion, i.e., Con) c) use experimental data for the evaluation of force constants for simple molecules d) interpret the differences in vibrational spectra of species in the gas phase and frozen at approximately 77'K e) instruct the student in the use of the infrared spectrometer f ) acquaint the student with cryogenic techniques
The species suggested for study are CO2 (C,J, SO2 (C2.), NzO (C,,), and CH2Cla ( C d , each providing a different degree of complexity. The species chosen and the experimental techniques employed would depend upon which of the objectives the instructor deemed important within the framework of his particular course.
One of the materials suggested in this experiment is a liquid (CH,CI,) which has a fairly high vapor pressure a t room temperature. Liquids are usually handled by first freezing with liquid N2 and then evacuating the air above the frozen sample. After evacuation, the frozen sample is warmed to room temperature and a stopcock may be used as a pressure controlling valve for delivering the desired amount of gas into the infrared cell. Spectra of Gases Frozen at 77'K
Many areas (nmr, esr, calorimetry, etc.) of research in physical chemistry today are involved in low temperature studies (cryogenics). I n this portion, we will become familiar with a low temperature technique used to study the infrared spectra of frozen gases. This same general technique is applied to the study of matrixisolated species. The heart of the experimental apparatus is the low temperature cell. It is a modified Hornig-type cell2and is shown in Figure 1. After assembling the various parts, the cell is evacuated to about 10P or torr with a n ordinary mechanical pump. The copper block and ir transmitting window are cooled by pouring liquid Na down the stem. This must b e done slowly
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43-
Experimental Spectra of Gases at Room Temperature
Presently, most physical chemistry laboratories are equipped with some type of gas handling vacuum system. I n its simplest form, it may consist of a vacuum pump, a liquid N2trap, a monometer, and a few vacuum stopcock inlets. A typical system has been described.' Three of the materials (C02,SO2,and N20) suggested in the present experiment are gases contained in commercially available lecture bottles and are delivered to the gas infrared cell using standard manometric procedures. The cell may be adapted with either NaCl or KBr windows which transmit infrared radiation down to approximately 600 cm-I and 400 cm-I, respectively. The infrared spectrum of the gaseous material is recorded after the student is taught proper use of the instrument by the person in charge of the laboratory. GUILLORY, W. A., J. C H E MEDUC., . 44,511 (1967). D. F., J. Chem. Phys., 18, WAONER,E. L.,AND HORNIG, 296 (1950).
%lo
.-4 F. FRONTVIEW
STEM
Figure 1. Diagram of the low-temperaluro Hornig-type cell. Front View: A, 5 5 / 5 0 gmund ioint; B, 8 3 0 O-ring joint; C, ir window; D, deposition jet. Stem: A, 5 5 / 5 0 ground ioint; 8, liquid N1 receptacle; C, a/,.in. Kovar real; D, Cu block; E, ir window; F, ring screw. Side View. A, to pump; B, 14/35 gmund joint; C, sample deposition jet; D, O-ring groove 45-rnm dim; E,clear view 30-mm dim.
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so that the ir window will not crack. If the window does crack during the cooling process, i t will have very little effect upon the resulting spectrum. When the copper block and window are cooled to approximately liquid Na temperature (liquid N2 ceises to boil away vigorously and collects in the stem), the stem is rotated from the top so that the gas being studied may be deposited directly onto the cold window. Approximately 2-4 torr of gas is deposited from the gas handling manifold (about 1-2 1 volume) over a 5-min period. The stem is then rotated by DO0 so that ir radiation can be transmitted through the deposition window. The low temperature cell may now be isolated under vacuum, detached, and placed in the sample beam of the infrared spectrometer. The spectrum is recorded as in the case of the gas cell. The liquid N2 receptacle usually requires some replenishing while the spectrum is being recorded. Results and Discussion Spectral Interpretofion
The infrared spectra of NaO, SO2, and CHzClz a t room temperature and 77'K are shown in Figures 2, 3, and 4, respectively. These spectra were recorded on a Perkin-Elmer 21 spectrophotometer which is used in conjunction with the physical chemistry laboratory in this department. Some very general guidelines are suggested for the interpretation of infrared spectra a ) In general, the more intense absorptions are probably due to fundamental vibration frequencies ("allowed" by the vibrational selection rules Au = +I). b ) For a given spectrum, the higher frequency absorption fundamentals are usually due to stretching motions and the lower fundamentals due to bendingmotions. c) For linear molecules, parallel vibrations have no Q branch (exception NO), but perpendicular vibrations do. d ) Overtone transitions may occur with decreasing intensity as A" gets larger ("forbidden" by first-order vibrational selection rules, Au = 1 2 , *3, etc.). e) Combination and difference transitions result from the simultaneous change of one quant,a from two different vibrationd
Figure 3. a, Infrared spectrum of 3 0 mm of gaseous SOX in o 10-cm cell. b, Comparison spectrum of pure SO1 deposited on a m l d window at approximately 77'K.
modes. Combinations are generally more intense than difference transitions since significant population of at least the first vihrational state (u = 1) is required for the latter.
Following these general guidelines, the interpretation of the NaO gas spectrum yields the following results Obserued Frequacies (nn-') 1165 (w.)
1285 (s.)
Orisin 1165 (overtone)/2 = 585 em-' bending fundamental t
(I)" + stretching fundamental N=N=O 1
..---
N = N 4 (11)' 2225 (v.s.) 2450 (v.w.) 2570 (w.) 3515 (w.)
-
stretching fundamental &
&
N = N d (11) 1165 f 1285 = 2450 cm-' comhination 1285 X 2 = 2570 cm-' overtone 1285 2225 = 3510 cm-I comhination
+
Some general observations of the low temperature spectrum with respect to the gaseous one reveal the following a) The absorptions are sharper with an observable shift in the hand center. b) The stretching fundamental absorptions have all but lost their P, Q, and R band shape. c) The extinction coefficient of the 1285 om-' fundamental has fundaineremed significantly with respect to the 2225 mental. Spectral Changes o f Frozen Films
The infrared spectral changes which occur when frozen films are compared to gas phase observations Figure 2. a, Infrared spectrum of 5 0 mm of gaseous NIO in a 10-cm cell. b, Comparison spectrum of pure NzO deposited on 0 m l d window a t opproximotely 779K.
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Perpendicular hand; motion of the electric dipole perpendicular to the molecular axis. 'Parallel band; motion of the electric dipole parallel to the molecular axis.
stants may be completely theoretical or semiempirical. The former has been accomplished for very few simple molecules, and therefore, the latter is more commonly employed. The quadratic potential energy function for NaO is 2V
=
knQ?
+ baQz2 + knQ? + kuQ~Qz+ kuQ,Qz + kzaQzQa
where the Q's represent the displacements of bond lengths and angles from their equilibrium positions. The k's arc the restoring force constants characterizing each type of vibration motion. Making the rather drastic approximation that the interaction force constants are zero (each vibration occurs independently and is unaffected by the others), the potential function reduces to the valence force field expression 2v = k , ~ , = b ~ ? !QQ? (2) corresponding to Q1, N=N displacement, Qz,N=O displacement, and &a, the bending mode. The combination of the valence potential function (eqn. (2)) and the quadratic expression of the lcinctic energy function leads to a detcrminantal equation, which yields the following solutions"
+
A 2a
Figure 4. o, Infrared spectrum of 40 mm llowest trocel of goreour CHnClz in a 10-cm cell. Decreasing the pressure progressively to 16, 6, and 2 does not resolve the 750 cm-' region obrorptionr, whereas rnm tho rpcctrvm of the fmren gas ot 77'K doer, b.
result primarily from two factors; the loss of rotational freedom and the increased density allowing greater interaction. The loss of quantitizcd rotational levels means that vihrational transitions have no rotational structure and thus a given absorption is not distributed over a wide energy range. The result is a sharper absorption with an apparent increased relative intensity with respect to the gas phase absorption hand. Another result which follows is the loss of the P, &, and R band shape, which occurs in the gas because of the Boltzmann distribution over the rotational levels. The other factor, which has to do with the close proximity of the molecules, results in a resonance type interaction between the vibrational levels of different molecules. Thus, one observes an average absorption center which may differ significantly from the hand center of the free gas phase molecule. I n addition, the closeness of the molecular species to each other results in a distortion of their electrical distribution. Since the intensity of a vibrational transition is proportional to dp/dq, any change in this derivative in going from the gas to the condensed state will be reflected in the absorption intensity. Occasionally, two fundamental (vibration) ahsorptions will overlap in the gas phase making it difficult to characterize the hand of the weaker absorption. This problem is often resolved by recording the spectrum of the condensed film at 77'K (Fig. 4, 650-800 cm-' region). Potential Fundions and Evaluation of Force Constants
One studies vihrational spectra of polyatomic molecules in order to understand the forces which hold the nuclei of a molecule in their equilibrium positions. The approach one may use to obtain these forcc con-
(1)
+
These solutions relate the frequencies (v's) to the atomic masses (m's), the bond distances (v's), and the unknown force constants of the molecule. From the experimental data, v1 = 1285 ~ m - ' , ~ v=~ 585 cm-', and v a = 2225 cm-I; 7Ni.q = 1.13 A and rNo = 1.19 A are obtained from microwave data. The calculated force constants are 14.6, 13.7, and 0.49 millidynes/A for kl, kz, ka/~NNvNo,respectively, as compared with 17.88, 11.39, and 0.49 millidynes/A when the klzQ1Q2 term is included (eqn. (2)). The solutions for calculation of the force constants for the other suggested molecules can also be found in Herzherg! Finally, one can relate the magnitude of the forcc constants to the bond order and correspondingly the bond strength. However, this is only true for rather isolated bonds; in the case of N20, the strong interaction between the two stretching modes makes this correlation unrealistic. Acknowledgments
The author gratefully acknowledges Drs. Jon Hougen and Dolphus Milligan who read the manuscript and made helpful suggestions. HEREBERG, G., "Infrared and Raman Spectra," (12t,h printing), D. Van Nostrand Co., Ine., New York, 1945, pp. 173-5.
See footnote 5.
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