2204
J . Phys. Chem. 1989, 93, 2204-2209
ARTICLES Spectroscopy of High Vibrational Levels of the N-N Stretching Mode of N-N-''O N-N-~~o
and
N. L. S. Yamasaki, Carlos Manzanares I., L. C. Baylor, G. C. Schatz,* and.Eric Weitz* Department of Chemistry, Northwestern University, Evanston, Illinois 60201 (Received: February 16, 1988; In Final Form: July 19, 1988)
The fundamental and overtones up to L' = 6 of the u, mode of N2I60and N2180have been studied. The positions of the u3 overtones can be fit by a tweparameter model since there is very limited interaction with other molecular vibrations. Vibrational self-consistent field and configuration interaction calculations and isotopic shift data show that the N-N stretching overtone is best described as a normal mode. Both N2I60and N2'*0 display line widths for the R branch of the v = 6 overtone of less than 2 cm-I.
Introduction The fundamental and first overtone frequencies of the u3 (N-N stretch) vibrational mode of N 2 0 have long been known.'-5 However, there is only one prior study of the higher u3 overtones.6 The small dipole moment derivative matrix elements5s7 and therefore the low absorbances of these higher energy transitions may account for this noticeable gap in information on a very well-studied molecule. Indeed, the study of ref 6 was performed in the photographic infrared and reports very weak intensities for some transitions even at absorption path lengths up to 4500 matm. The higher sensitivity of photoacoustic (PA) spectroscopy may allow for a more accurate determination of overtone positions and can, at least in principle, measure spectral line widths withopt significant pressure broadening. This molecule provides for an interesting study of the behavior of vibrational overtones in a small non-hydrogen-containing molecule. The lowest vibrational overtones have been studied for many different molecules [e.g., ref 1, 8, and 91, but except for the PA study of CF2C12 ( n I 5)1° and the recent work in this laboratory on C z C overtones," only X-H type (X = C, 0, N ) highly excited vibrations have been s t ~ d i e d . ~ Theoretical .~~~~~'~ workI4J5 suggests that local-mode theory is most applicable to overtones involving a large mass difference between the vibrating moiety and the rest of the molecule. Thus, the lightness of the hydrogen atom as compared to the mass of the X atom in an X-H bond is a major factor in determining the behavior of an overtone.16 The behavior of the overtones of the N-N stretch in N,O could (1) Herzberg, G. Infrared and Raman Spectra; Van Nostrand: Princeton, NJ, 1945. (2) Margolis, J . S. J . Quant. Spectrosc. Radial. Transfer 1972, 12, 751. (3) Amiot, C. J . Mol. Spectrosc. 1976, 59, 380. (4) Chedin, A.; Amiot, C.; Cihla, Z. J . Mol. Spectrosc. 1976, 63, 348. (5) Kagann, R. H. J . Mol. Spectrosc. 1982, 95, 297. (6) Herzberg, G.;Herzberg, L. J . Chem. Phys. 1950, 18, 1551. (7) Suzuki, I . J . Mol. Spectrosc. 1980, 80, 12. (8) Hayward, R. J.; Henry, B. R. J . Mol. Spectrosc. 1975, 57, 221. (9) Hayward, R. J.; Henry, B. R. Chem. Phys. 1976, 12, 387. (IO) Brabham, D. E.; Perry, D. S . Chem. Phys. Lett. 1984, 103, 487. (11) Manzanares I., C.; Yamasaki, N. L. S.; Weitz, E . J . Phys. Chem. 1986, 90, 3953. ( I 2) Fang, H. L.; Meister, D. M.; Swofford, R . L . J . Phys. Chem. 1984, 88, 410. (13) Wong, J. S.; Moore, C. B. J . Chem. Phys. 1982, 77, 603. (14) Child, M. S.; Lawton,-R. T.Faraday Discuss. Chem. Soc. 1981, 7 1 , 213. Child, M. S. Arc. Chem. Res. 1985, 18, 45. ( 1 5 ) Wallace, R . Chem. Phys. 1975, 11, 189. (16) Henry, B. R.; Hung,I . F . Chem. Phys. 1978, 29, 465.
0022-3654/89/2093-2204$01 S O / O
thus prove quite revealing. This study will describe N 2 0 spectral characteristics up to v = 6, enabling the comparison of non-X-H and X-H vibrational overtone behavior. In addition, overtone line widths are determined and analyzed. To characterize these overtones as either local-mode-like or normal-mode-like, we perform vibrational self-consistent field (SCF) calculations to determine the best effective vibrational wave function that can be expressed as a product of individual mode wave functions by using either local-mode or normal-mode coordinates. In addition, we use configuration interaction calculations (the vibrational analogue of electronic CI) to determine the relative importance of the local- and normal-mode-based S C F wave functions in the full wave function associated with each overtone.
Experimental Section The fundamental and first overtone spectra of the molecules under consideration were recorded in a 10-cm gas cell with a Nicolet 7 199 FT-IR spectrophotometer. One hundred scans were averaged at a resolution of -2 cm-I. The lower overtones (L' = 3 and v = 4) were recorded on a Perkin-Elmer 330 (PE-330) UV-vis-near-IR spectrophotometer at a resolution of -2 nm. The first overtones were obtained in a standard gas cell that was 10 cm in length and 2.5 cm in diameter and fitted with NaCl windows. However, the higher overtones exhibited considerably smaller absorption cross sections. Thus a Wilks variable long-path (0.75-25.75 m) gas cell equipped with quartz windows was employed. In some cases, improvement was also achieved by averaging up to 15 scans of a spectral region by means of a Perkin-Elmer data station. The spectra in the o' = 6 region were observed by intracavity dye laser photoacoustic spectroscopy. The PA cell and experimental apparatus have been described previously." Briefly, the cell is mounted on a rotational-translational platform within the cavity of a continuous wave dye laser (Coherent, Model CR599-01) that is pumped by a krypton ion laser (Coherent, Model CR-3000K). For the overtone region scanned (L' = 6), the dye, LD 700, is pumped with 5 W of multiline red krypton ion laser output. This dye lases over the range 695-805 nm. The dye region was scanned at -0.5 cm-' resolution by tuning a three-plate birefringent filter driven by a microprocessor-controlled motor. Absolute frequencies were determined with a calibrated Instruments SA Model HR320 monochromator and an RCA-1P28
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(17) Manzanares I., C.; Yamasaki, N. L. S.: Weitz, E.: Knudtson, J. T.
Chem. Phys. Lett. 1985, 11 7, 471.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
Spectra of N2160and N2I80
.osoj .040
TABLE I: Gas-Phase Fundamental and (cm-I) for N 2 0 and N2'*0 vibratnl species this study level u
I
N20 N2180 N20 N2I80 N2O N20 N2O N20
N2I80
6
3 .020 4500
4450
4400
43150
43100
