J. Phys. Chem. 1082,86,664-670
664
Vibrational Spectra and Normal Coordinate Analysis of CF, Compounds. 37.'12 Molecular Structure, Vibrational Spectra, and Normal Coordinate Analysls of the Perfluoromethylamlnes CF,NF, and (CF,),NF H. Oberhammer;
H. Gunther,
Instnut fur Physikalisctw Chemle der Universitiit, P7400 Tibinget?, West Germany
H. Burger, F. Heyder, and 0. Pawelke FB 9 Anorgnlsche Chemie, Gesambrrochschuie Unhrerslt, Wuppertal D-5800 Wuppertal 1, West Germany (Received: Ju& 13, 1981; In Final Form: September 14, 1981)
The geometric structure of CF3NF2has been determined by joint analysis of electron diffraction and microwave spectroscopy data. For (CFd2NFthe structure determination is based on electron diffraction intensities alone. The b a n and IR spectra of both compounds have been recorded in the liquid and gaseous state and assigned for C, symmetry. Normal coordinate analyses have been performed by making use of extensive transfer of force constants in the series (CF3)"NFSn. In this series the N-F bond lengths do not depend on the number of CF3groups within the experimental error limits. A systematic shortening of the N-C bond lengths and an increase in the corresponding force constants with increasing number of CF3 groups is observed. An attempt is made to rationalize this trend on the basis of a simple bonding model.
Introduction The geometrical structures at the nitrogen atom of NF3 and N(CF3), are very different, although the chemical behavior, e.g., stability, lack of Bronstead and Lewis basicity, are quite comparable. While the bond angle at the nitrogen atom of NF3 is extremely small, 102.2 (1)",3 this angle is widened to 117.9 ( 4 ) O in (CF&N.* This flattening of the pyramid at nitrogen has been ascribed to F-F repulsions between different CF3 groyps. We felt that it was desirable to study this striking effect by extending the previous investigation of N(CF3),4to the molecules in between, CF3NFzand (CF3)2NF.Neither has been investigated structurally, and the vibrational spectra of both species are not known except for an incomplete IR study of CF3NF25and a listing of several of its IR absorptions.6 In the following we report the structures of CF3NF2and (CF3)2NFas determined by electron diffraction and microwave spectroscopy and their infrared and Raman spectra in the gaseous and the liquid phase. The vibrational behavior is described quantitatively by a normal coordinate analysis in which particular attention is given to the transferability of force constants in the series (CF&nNF3-n* Experimental Section CF3NF2and (CF3),NF were obtained as byproducts from the electrochemical fluorination of N(CH3)34g7 and purified by repeated low temperature distillation through (1) Part 36: K. Burczyk, H. Biirger, A. Ruoff, and M. Morillon, Spectrochim. Acta, in press. (2) Part 35. R. Eujen, H. Biirger, and H. Oberhammer,J. Mol. Struct., in press. (3) M. Otake, C. Matsumura, and Y. Morino, J. Mol. Spectrosc., 28, 316 (1968). (4) H. Biirger, H. Niepel, G . Pawelke, and H. Oberhammer,J. Mol. Struct., 54, 159 (1979). (5) R. H. Atalla and A. D. Craig, J. Chem. Phys., 45, 427 (1966). (6)C. W. Bjork, N. C. Craig, R. A. Mitsch, and J. Overend, J. Amr. Chem. Soc., 87, 1186 (1965). (7),H.Biirger, H. Niepel, G. Pawelke, H. J. Frohn, and P. Sartori, J. Fluorme Chem., 15, 231 (1980). 0022-365418212086-0664$0l.25/0
TABLE I: Results of Microwave Spectroscopy Analysis (Ir Representation)" Rotational Constants. GHz
Bo! B,! BZ1(calcd)
A
B
C
3.754548(9) 3.75329(19) 3.75341
2.627619 ( 4 ) 2.62573(28) 2.62586
2.140133 ( 4 ) 2.13901 (17) 2.13913
Centrifugal Distortion Constants, kHz AJ
AJK
AK
65
6K
expt 353 (35) 3 3 2 ( 9 3 ) - 5 8 ( 2 3 6 ) 71 ( 6 ) -673 ( 7 1 ) calcdb 359 358 -125 77 -588 Dipole Moments, D
Pu,=o.6(2)
pb=o.15(5)
pbt=0.6(2)
Quadruuole Coupling Constants. MHz
1.02 ( 8 ) Xbb = 5.98 (17) Xcc = - 7.00 (17) The principal axis system is indicated in Figure 3. Calculated from force field of Table IV. Xaa =
a
a slit tube column, bp -78 "C (CF3NF2)and -37 "C ((CF&JW. Electron Diffraction. The diffraction intensities were recorded on 18 X 13 cm Kodak electron image plates with the Balzers gas diffractorgraph KD-G28 at two camera distances (50 and 25 cm). During the experiment the samples were kept at -106 "C (CF3NF,) and -84 "C ((CF3)2NF), respectively. The nozzle temperature was ca. 10 OC in all cases. The camera pressure never exceeded 5 X torr. (Throughout this paper 1 A = 100 pm and 1 torr = 101.325/760 kPa.) The electron wavelength was determined from ZnO diffraction patterns. The s-range covered in the experiment was 1.4-17 A-' (long distance) and 8-35 A-' (short distance). For each camera distance two plates were selected and evaluated in the usual way.s For the short camera distance, intensities recorded without (8) H. Oberhammer, "MolecularStructures by Diffraction Methods", Vol. 4, The Chemical Society, Burlington House, London, 1976, p 24. (9) H. Oberhammer, H. W h e r , and W. Gombler, J.Mol. Struct., 70, 273 (1981).
0 1982 American Chemical Society
The Journal of Physlcal Chemlstty, Vol. 86, No. 5, 1982 605
Vibrational Spectra of CF3
e a
4'
c
2 0 3
c 4 2
Figure 1. Experimental (0) and calculated (-) and differences for CF3NF,.
( e 2
s
3 2 a - '
molecular intensities
c
4 '
2
2
20 3
ZA
e
re
F-
.-32 .I
Flgure 2. Experimental (0) and calculated (-) and differences for (CF,),NF.
a \ r 3
1
molecular intensities
--
1
2 0 R
3.0
4
0
5.0
6
0
C A n g s t r o m Z
Flgure 3. Experimental radial distribution function and dmerence curve for CF,NF,.
gas were subtracted. The averaged molecular intensities for both camera distances are presented in Figures 1and 2,1° and the radial distribution functions are in Figures 3 and 4. Microwave Spectra. The rotational spectrum for CF3NF2 was recorded with a conventional Starck spectrometer'' in the frequency range 9-40 GHz at a temperature of -50 "C. A Starck field of 200-1600 V/cm with 100-kHz frequency was applied. Only a-type transitions have been assigned. 39 R branch transitions from J = 1 2 to J = 8 9 and 25 Q branch transitions from J = 16 to J = 66 were measured. A complete list of experimental and calculated transitions is available as supplementary material.1° In the centrifugal distortion analysis only quartic
-
1 0
2
0
3
0
1
5 0 9 CA-gs,trom? 4 0
Flgure 4. Experimental radial distribution function and difference curve for (CF3),NF.
P,7
1 0
-_ _
\13 1
-
(10) See paragraph at end of text regarding supplementary material. (11) H. D. Kamphusmann, Ph.D. Thesis, University of Ulm, 1970.
terms of the angular momentum operators were considered. The results of a least-squares fit are summarized in Table I. The principal axes are indicated in Figure 2. The centrifugal distortion constants are compared to the values calculated from the force field of Table IV. The dipole moment was estimated from Starck splitting. A more accurate determination would require an exact treatment of the energy matrix including rotational and electrostatic terms. The quadrupole coupling constants were calculated from the hyperfine structure of 13 transitions. Due to the high value for x- (12.98 MHz), hyperfine splitting for Q branch transitions with high J values could still be observed. The torsional frequency of the CF, group is estimated to 110 (20) cm-' from the relative intensities of u = 0, 1, 2, and 3 transitions. Attempts were made to measure the microwave spectrum for (CF&NF. Some very weak lines, indicating a small dipole moment, were observed but could not yet be assigned. On the basis of the electron diffraction structure b- and C-type spectra are expected. Vibrational Spectra. Gas-phase IR spectra were recorded by employing 10-cm gas cells fitted with KBr and polyethylene windows. For the 4000-200-cm-' region a Perkin-Elmer type 580B spectrometer was employed, while the FIR spectrum in the 400-80 cm-' range was recorded with a Nicolet Series 8000 vacuum FT spectrometer: resolution 0.8-1.5 cm-', wave number accuracy better than 1 cm-'. In addition, some absorptions of CF,NF2 in the mid-IR were studied with a resolution of 0.12 cm-' by employing a Nicolet Series 7199 FT instrument. Raman spectra of CF3NFzwere recorded with a Cary 82 instrument, Ar+ 4880-A excitation. The gas phase was studied at a pressure of 1500 torr by using a multipass device, 1.5 W at the sample, while the liquid phase was investigated at -65 "C. The spectrum of (CF3),NF in a sealed 1-mm i.d. capillary was recorded with a Cary 82 instrument with Kr+ 6471-A excitation. Spectral slit widths were 1.5-3 cm-', and the wave number accuracy was 1-2 cm-', improving to
