3953
J. Phys. Chem. 1986,90, 3953-3958
Overtone Spectroscopy of 3,3,3-Trifluoropropyne Carlos Manzanares I,+* N. L. S. Yamasaki, and Eric Weitz* Department of Chemistry, Northwestern University, Evanston, Illinois 60201 (Received: January 8, 1986)
Intracavity dye laser photoacoustic spectra have been obtained for overtone excitation of the C-H stretch (Aul = 4, 5, and 6) of gas-phase CF3C=CH. Standard infrared techniques were used to obtain the fundamental and lower overtones (Aul = 1, 2, 3). The local-mode model is used to assign prominent peaks. For all transitions in the C-H stretching mode a band close in energy to the main absorption is observed and is assigned as a hot band (vi = 0, u7 = 1) ( u l = u, u7 = 1) for u = 1-6, which shows the strong interaction between the C-H stretching mode (vl) and the C-H bending mode (v,). Also the fundamental and lower overtones (Au2 = 1, 2, 3) of the CEC stretch are obtained. Harmonic frequencies (q)and anharmonicity constants (&) with i = 1 or 2 for C-H and CEC bonds, respectively, are calculated. The off-diagonal local-mode-normal-mode anharmonicity constant X I , derived from the interaction between modes v1 and v7 is also reported. Peak absorption cross sections are obtained for u I = 1-5 of CH absorptions and uI = 1-3 of C z C absorptions.
-
Introduction The absorption spectra in the infrared, near-infrared, and visible regions of the electromagnetic spectrum of compounds containing isolated C-H stretching modes have attracted much interest in recent years. Studies of this type have resulted in the formulation of the local-mode picture for high-lying vibrations and have been used to test model theoretical calculations and also to provide information about vibrational energy redistribution in polyatomic molecules. In saturated compounds such as CF3H, CD3H, and (CF,),CH it has been found1+ that there is a universal anharmonic interaction in which the C-H stretching and the overtone of the C-H bending vibrations are coupled by a strong Fermi resonance. Studies of C-H stretching fundamental and overtone transitions in fluoroacetylenic compounds such as (CF3)3CC==CH and CF3C=CH have shown that these transitions are less strongly coupled by resonance interaction with vibrational bending modes than the same transitions in saturated compounds such as CF3H and (CF3)3CH. In fact, it has been ~ b s e r v e dthat ~ , ~ in CF3C=CH Fermi resonance does not occur between the C-H stretch and the overtone of the bend due to the large mismatch in frequency between twice the bending mode and the fundamental of the C-H stretching mode. Instead, the C-H vibrational absorption regions are dominated by discrete transitions not strongly perturbed by Fermi resonance interaction. In addition to C-H stretching transitions, in general these are sum and difference bands of the excited C-H overtone and the chain-bending fundamental ( u I 0 ) or the C-H bending fundamental (v,). Other vibrational structure in CF3CCH has been discussed5 that corresponds to hot-band transitions involving the C-H stretching ( u l ) and the chain-bending modes ( v l 0 ) producing a sequence of bands in the fundamental (vl = 0, vl0 = m ) (ul = 1, ul0 = m) m = 0, 1, 2, 3, ... . The same sequence is found for the first and second overtone of the CH absorption.6 An important relevant observation of these studies5v6is the identification in the fundamental spectrum of the strongly shifted hot band originating in the v7 mode (686 crn-') that appears in the spectrum a t 3310.35 cm-I. The appearance of this band indicates a strong nonresonant intramolecular coupling between the C H bending (v,) and the C H stretching ( v l ) fundamentals. The vibrational assignments for and spectrum of 3,3,3-trifluoropropyne (shown in Table I) have been reported, and its microwave spectrum and molecular constants have been obt a i ~ ~ e d . ~ - "The band shape of the infrared band due to the v2 acetylenic stretch has been investigated1*through observation of its temperature dependence and by matrix isolation studies. In this paper the gas-phase overtone spectrum of the C H stretching mode in 3,3,3-trifluoropropyne is presented for Aul = 2, 3, 4, 5, and 6 . In addition, a hot-band transition of the type (u, = 0, v7 = 1) (u, = u, v7 = 1) is identified for u = 1-6. This
-
-
'Permanent address: Departamento de Quimica, Universidad Simbn Bolivar, Apartado 80659, Caracas 1081, Venezuela.
0022-3654/86/2090-3953$01.50/0
TABLE I: Observed Fundamental Frequencies of C F , m H " assignment freq, cm-I al Modes VI C-H stretch 3328.1 v2 C=C stretch 2165.4 v3 C-F stretch 1253.2 v4 C-C stretch 811.7 VS CF, deformation 536.1 e Modes v6 C-F stretch 1179.2 Vl CsC-H bend 686 V8 CF, deformation 611.9 v9 CF, rock 453 VI0 C-C=C bend 171 "Reference 7. strengthens the supposition of strong nonresonant intramolecular coupling between the C H stretching ( v I ) and CH bending (v7). Bands corresponding to the C=C stretching fundamental (vz) and its lower overtones are also reported. Experimental Section A detailed description of the experimental approach has been previously p r 0 ~ i d e d . l Briefly, ~ laser photoacoustic spectra were obtained for the Aul = 4,5, and 6 C-H stretching vibrations with a cell mounted within the cavity of a continuous wave dye laser. The photoacoustic cell was constructed of a 20 cm length of 1-cm-diameter Pyrex tubing with the ends cut at the Brewster angle, and quartz windows attached with Torr seal cement. The signal was detected by a Knowles BT1759 electret microphone attached to a flange mounted a t the midpoint of the cell. The laser system consisted of a Kr+ ion laser, Coherent Radiation Model 3000 K, and a Coherent Model 599-01 dye laser with high-reflectance optics. The ion laser pump beam was modulated by a mechanical chopper at a frequency of 125 Hz. Wavelength (1) Dubal, H. R.; Quack, M. J. Chem. Phys. 1984,81, 3779. (2) Peyerimhoff, S.; Lewerentz, M.; Quack, M. Chem. Phys. Lett. 1984, 109, 563. (3) Baggot, J. E.; Chuang, M. C.; Zare, R. N.; Diibal, H. R.; Quack, M. J . Chem. Phys. 1985, 82, 1186. (4) Diibal, H. R.; Quack, M. Chem. Phys. Lett. 1980, 72, 342. (5) von Puttkamer, K.;DObal, H. R.; Quack, M. Faraday Discuss. Chem. Soc. 1983. 75. 197. (6) Diiba1,'H. R.; Quack, M. Chem. Phys. Lerr. 1982.90, 370. (7) Berney, C. V.; Cousins, L.R.; Miller, F.A. Spectrochim. Acra 1963, 19.,~ 2019. .. (8) Anderson, W. E.; Trambawlo, R.; Sheridan, J.; Gordy, W. Phys. Rev. 1951, 82, 58. (9) Mills, I. M. Mol. Phys. 1969, 16, 345. (10) Shoolery, J. N.; Shulman, R. G.; Sheehan, W. R.; Shomaker, V.; Yost, D. M. J. Chem. Phys. 1951, 19, 1363. (11) Grenier-Besson, M. L.;Amat, G. J . Mol. Spectrosc. 1962, 8, 22. (12) Sanborn, R. H. Spectrochim. Acta, Part A 1967, 23A, 1999. ~
0 1986 American Chemical Society
Manzanares I et al.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
3954
1 v(C-H)3 9682.3 crn-'
v(C-H)=6557.4cm-' v=2
V'3
.OS6
.028
I
I
3450
3350
Wovenumbers ( c m - ' )
i 32:o
O'
I
I
6600
6500
0 6400
I
I
I
9800
9700
9600
Wovenumbers (cm")
9500
Wovenumbers (cm-' )
Figure 1. Infrared spectra of the C-H absorption in the region around levels u I = 1, 2, and 3. u1 = 1 was taken in a 20-cm cell at 4.5 Torr with a Nicolet 7199 FTIR. u1 = 2 and 3 were taken with a Perkin-Elmer 330 spectrophotometerand a Wilkes variable path-length cell. The path length was 0.75 m for the ut = 2 and 3.75 m for the v1 = 3 level, both at 160 Torr.
tuning (0.5 cm-I bandwidth) of the dye laser was accomplished with a motor-driven birefringent filter. The motor was controlled by a microprocessor. Wavelengths were calibrated with a JY HR320 monochromator. Signals from the microphone were amplified and processed by an Jthaco lock-in amplifier Model 391A. The laser output was monitored with a photodiode and a PAR Model 128A lock-in amplifier. Normalization of the photoacoustic spectra was achieved by ratioing the output signals from both lock-in amplifiers. The normalized signal was displayed on a strip-chart recorder. The tuning ranges of the laser dyes were as follows: LD 700 (12 000-1 4 000 cm-') pumped by all red lines of the Krf laser and Rhodamine B (14800-16500 cm-l) and Rhodamine 110 (17000-18 600 cm-') pumped by the blue-green lines of the Kr' ion laser. In each case a high-reflectance (>99.7%) dye laser output coupler was used to increase intracavity laser power. The room temperature gas-phase overtone spectra for C H and CC, Au = 2, 3, were obtained with a Wilks variable path-length gas cell (Wilks ScientificCorp., Model 5720) and a Perkin Elmer IR-UV-vis spectrophotometerModel No. 330. The fundamentals for C H and CC were obtained with a Nicolet 7199 Fourier transform infrared spectrophotometer and a 10-cm path-length cell. 3,3,3-Trifluoropropyne ( C F 3 m H ) specified as 99% pure was obtained from SCM Specialty Chemicals and used without further purification. All experiments were run at 21 f 2 OC. RMdts
The spectra shown in Figure 1 correspond to transitions to levels 1, 2, and 3 of the C-H stretching vibration. The fundamental was obtained with a Nicolet 7199 FTIR and a 10-cm cell at a pressure of 4.5 Torr. Transitions with Au, = 2 and 3 were obtained with a Perkin-Elmer 330 spectrophotometer and the Wilkes variable path-length cell. The path length was 0.75 m for Aul = 2 and 3.75 nm for Aul = 3, both at a total pressure of 160 Torr. Figure 2 shows overtone spectra corresponding to transitions Aq = 4,5, and 6 of the C-H vibration obtained with the intracavity dye laser photoacoustic technique. Pressures for Avl = 4, 5 , and 6 were 500, 104, and 705 Torr, respectively. Transitions with Auz = 1, 2, and 3 of the C=C vibration were also obtained and are shown in Figure 3. The fundamental was obtained with a IO-crn
TABLE II: Observed and Calculated Frequencies (ern-') for Gas-PhaseCF,C=CH spectral region obsd freq hi(C-H)
AuI(C-H)
1
2
AuI(C-H)
=3
AuI(C--H)
4
AuI(C-H)
=5
AuI(C--H)
=6
Au~(CEC) 1 AU2(C=C) = 2 A u ~ ( C E C )= 3
3310.4 3319.4 3328.1 3336.3 6522.5 6557.4 6725 7204.6 9625.3 9682.3 12516.6 12601.3 12684 12850.1 15534.5 15640.5 18391 18516 2165 4303 6386
calcd freq"
assigned transition
3325.9 6553.6
9683.1 12647.5 15647.5 18482.4 2167 4298 6393
"Calculated by use of eq 3 and the values of w, and XI, in Table 111. path-length cell at 4.5 torr using an FTIR spectrophotometer. For Au2 = 2 and 3 the Perkin-Elmer 330 spectrophotometer was used. The first overtone was obtained with a IO-cm cell at a total pressure of 280 Torr and the second overtone with the variable path-length cell at a pressure of 160 Torr and a path length of 6.75 m. The observed and calculated energies with their respective assignments for selected transitions of the CF,CCH molecules are shown in Table 11. The peak absorption cross sections for the different C-H and C=C vibrational levels are listed in Table 111. For C-H absorptions and quantum levels u1 = 4 and 5, the values are determined by the use of an internal standard of known cross section and the equationi3 u
= u,VPo/V,P
(1)
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3955
Overtone Spectroscopy of 3,3,3-Trifluoropropyne
I
v(C-H)=12684 cm-' v:4
v(C-H)= 15640.5cm-I
v =5
i I
I
12800
I
I
I
I5 800
I2600
15600
I
I
I
.
I8600
15400
I
18400
I
I
18200
Wovenumbers (ern-') Wavenumbers (cm-') Wove nurn be r s (c m Figure 2. Intracavity photoacoustic spectra of the C-H absorption in the region around levels u, = 4,5, and 6. Experimental conditions are described
in the text. ,012
"['
,070
.22 V ( C n C ) = 4303cm" ,056 V I2
,042
,028
.014
b
I
2250
2150
0 3
4,
I 4300
4100
"
6;50
6350
I,
6250
Wavenumbers (ern-') W a venum b e r s (c m-' 1 Wavenumbers (ern-') Figure 3. Infrared spectra of the C=C absorption in the region around levels u2 = 1, 2, and 3. u2 = 1 was taken in a 20-cm cell at 4.5 Torr with a Nicolet 7199 FTIR. u2 = 2 was taken with a Perkin-Elmer 330 spectrophotometerand a IO-cm cell at 280 Torr. v2 = 3 was taken with the same spectrometer and a Wilkes variable path-length cell. The path length was 6.75 m at 100 Torr.
where P is the pressure in Torr, Vthe normalized signal intensity (arbitrary units), and u the absorption cross section in units of pm2/molecule. The subscript "0" identifies the internal standard molecule. The v1 = 4 cross section was determined relative to the u = 5 (C-H) cross section of methane; a0 = 1.4 X (13) Reddy, K.V.;Heller, D. F.; Berry, M. J. J . Chem. Phys. 1982, 76,
2814.
pm2/molecule. The v = 5 C-H absorption cross section was determined relative to the v = 6 (C-H)cross section of tetramethylsilane, which was previously determined;I4 a,, = 5.6 X lo4 pm2/molecule. Methane could not be used for the u1 = 5 determination because of the limited tuning range of the laser dye. (14) Manzanares I, C.; Yamasaki, N. L. S.; Weitz, E.;Knudtson, J. T. Chem. Phys. Lett. 1985, 1 1 7, 411.
3956 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
Manzanares I et al.
TABLE III: Peak Cross Sections ( u ) for CH and CC Absorptions, Harmonic Freauencies, and Anharmonic Constants u, pm2/molecule
c=c
C-H 16.9 2.47 X lo-' 6.26 x 10-3 1.32 X 4.93 x 10-5
c
12.0 1.3 X lo-* 2.0 x 10-3
harmonic frequencies, crn-' wi = 3375 i 9' anharmonic constants, cm-' xi,= -49 i 24 anharmonic interaction constant, X , , = -19 & 2"
w2 =
2185 i 12" i 55
x,,= -18
"0°-
Y, Y,
C - H sIreILh C * C - H bend
cm-'
Reported errors are 2u.
Quantum Number ( n i
No background signal was observed for an empty cell in regions of absorption of these molecules.
Figure 4. Birge-Sponer plots of C-H absorptions. AE/u, vs. ui for CF3C=CH and HC=N. Experimental points for HCN are from ref 16 and 17.
Discussion For a polyatomic molecule the local-mode representation of a vibrational energy level is usually expressed aslS
with the ones obtained in ref 6 using a fundamental and the lower overtones, Avl = 2 and 3. As a further test for the assignment of the low-frequency absorption as originating in a hot-band transition of CF,CCH, the results from another system (HCN) with an isolated C-H stretching mode were analyzed and compared with the present results. With experimental energies from ref 16 and 17, the CH stretch overtone absorptions 0 u3 of H C N and the hot bands (u, = 1, u, = 0) (u, = 1, u, = u ) with u = 1, 3, and 4 were plotted as ( A E / v 3 )and (AE'Iv,) vs. u,. The results are shown in Figure 4b. The upper line corresponds to the pure local-mode transition 0 u, and the lower line corresponds to the hot band (u2 = 1, u! = 0) (v2 = 1, u, = u ) . For H C N the diagonal anharmonicity constant X,,= -52.1 cm-l and the off-diagonal X 2 , = -(19.52 f 0.08) cm-' are very similar to the constants obtained for CF,CCH, where the same interactions between vibrational motions, C H stretching and CH bending, were considered. Also, as ref 5 points out, (CF,),CC=CH exhibits the same type of hot-band transition. These results suggest that this coupling between the C H stretch and C H bend of acetylenic compounds should be common to other similar molecules of the type RCECH. C-H Absorptions. Assignment of the most prominent features stretches will now in the overtone spectra of the C-H and be discussed. Lower overtones of the C-H stretching mode have been dealt with in detail in ref 2, 5, and 6. The C-H spectrum for Aul = 2 is shown in Figure 1 . As discussed before, the predominant peaks correspond to a pure local-mode overtone at 6557 cm-' and the hot band at 6522.5 cm-'. Other observed strong transitions (not shown) correspond, for example, to the combination bands, 2ul + u, at 7205 cm-I and 2ul + ulo at 6725 cm-I. The C-H bending fundamental u7 = 686 cm-I and the C-C-C chain bending fundamental uI0 = 170 cm-' have been founds to interact with the fundamental ul, creating a structure surrounding the main u1 transition and producing the transitions uI + uIo, u1 - vlo, v I v7, and ul - u7. The main absorption for Aul = 3 is at 9682.2 cm-I and also shows the smaller hot band on the lowenergy side of the spectrum at 9625.3 cm-' (see Figure 1). The Au, = 4 spectrum shown in Figure 2 exhibits three bands. The strongest one at 12 684 cm-' corresponds to the main C-H absorption, the middle one at 12601.3 cm-I corresponds to the hot-band transition, and the least intense of all is at 12 516.6 cm-I. This last absorption corresponds to the difference band 4ul - ul0. Also (not shown) the sum band 4ul + vlo was observed at 12850.1 cm-I. The overtone Aul = 5 (Figure 2) whose maximum absorption is at 15 640.5 cm-' exhibits the combination band 5ul u l o on the high-energy side at 15 819 cm-' and the hot-band absorption at 15 534.5 cm-I. The spectrum corresponding to Avl = 6 shown in Figure 2 shows clearly the hot band at 18 391 cm-'
where w i and u, are the harmonic frequency and vibrational quantum number, respectively, and w,,'s are harmonic coupling terms that are usually small and can be neglected. Besides the diagonal X I ,local-mode anharmonicities, off-diagonal XI, localmode-local-mode and local-mode-normal-mode anharmonicities are sometimes necessary to assign the spectra. With eq 2, the energy difference E(uJ - E(O), corresponding to a pure local-mode transition 0 u, is
AE = u,w,
-+
Similary for a hot-band transition (0, u,) difference E(u,,u,) - E(O,v,) is given by
AE' = uiwi
-
(3)
U,2XII
(u,, u,) the energy
+ v;Xii + uJij
(4)
Previous infrared absorption studies5s6on CF3CCH have shown that besides the PQR structure for the C H stretch fundamental centered at ul = 3328.1 cm-' there is a strongly shifted hot band originating in the u7 state at 686 cm-' that is centered at 3310 cm-'. This weaker band is shown in Figure 1 in the spectrum corresponding to the fundamental as a small shoulder at the low-frequency end of the P branch. The assignment of this band A similar lowfor the fundamental has been well establi~hed.~ frequency absorption is observable for the Avl = 2 and 3 transitions in Figure 1 and the Aul = 4, 5, and 6 transitions in Figure 2. The separation in energy between the maximum of main absorption and the low-frequency peak increases by a multiple of the vibrational quantum number of the excited level. With the experimental points for the C H stretch transitions 0 ul, a plot of A E l v , vs. u t was obtained and the results are shown on the upper straight line of Figure 4a. A similar plot was made with the experimental energies obtained for the small absorption on the low-energy side of each transition. A straight line was obtained from this data as shown in the lower line of Figure 4a. From this result it can be concluded that the weaker band, close to and at a lower frequency than the main overtone transition, is a hot band of the type ( u l = 0, u7 = 1) (ul = u, u7 = 1). The observed frequencies for these transitions are presented in Table 11. From the plots in Figure 4 and with eq 3 for the pure local-mode transition and eq 4 for the hot-band absorption, the harmonic frequency wl = 3375 f 9 cm-I, the diagonal local-mode anharmonicity XI, = -49 2 cm-I, and the off-diagonal local-modenormal-mode anharmonicity XI7 = -19 f 2 cm-' were obtained. Both sets of data in Figure 4 produced consistent values for XI and wl. The values for w Iand XI1are in very good agreement
-
-
*
(15) Henry, B.
R. Acc. Chem. Res.
1977, 10, 207.
-
-
--
+
+
(16) Douglas, A. E.; Sharma, D. J . Chem. Phys. 1953, 21, 448. (17) Lehman, K. K.; Scherer, G. J.; Klemperer, W. J . Chem. Phys. 1982, 77, 2853.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3957
Overtone Spectroscopy of 3,3,3-Trifluoropropyne
2200 2180 2160 7
-
2140
5 ;2120
4YI - 2 Y I o
; \
v)
L .-
2100
c
n
2080
L
O
2060
0
6
2040
n
2020
a
1
2
3
4
5
6
7
Quantum Number ( v 1 Figure 6. Birge-Sponer plot, AE/u2 vs. u2 for C=C absorptions of CF3C=CH. Experimental point (0) is from photoacoustic measurements. All others were obtained via conventional spectroscopy.
n 13000
12800
12600
12400
12200
12000
Wavenumbers (crn-l)
Figure 5. Photoacoustic spectrum of CF3C=CH in the region between 12 000 and 13 000 cm-I. The boxed spectrum is a 30-fold enlargement of the region between 12 500 and 12 100 cm-'.
and the main transition at 18 516 cm-I. The bands for Aul = 6 appear stronger as compared to Aul = 5 because the sample pressure and power of the dye laser were larger for Avl = 6. The structure on top of the main absorption for each photoacoustic spectrum shown in Figure 2 is probably due to a set of hot-band sequences that involve the main C-H stretch transition (vl) and hot bands of the C-C-C bending transition (vl0). These types of transitions have been studied and assigned by Diibal and Quack6 from high-resolution spectra of the fundamental and lower overtones (Av = 2 and 3) of CF,C=CH. The position of the 4vl absorption falls slightly off the straight line in Figure 4. This, in itself, is suggestive of a resonance interaction. However, from Table I it can be seen that there are no obvious binary resonances that could cause this shift. In addition, the line width of this transition is not broadened relative to the other vu1 transitions, as would be expected for a resonance. Thus, at present, the cause of the small deviation of 4vl from its expected position is not ascertainable. m C Absorptions. The band shape of the v2 acetylenic stretching fundamental is shown in Figure 3. The contour is much more complicated than the simple PQR structure displayed by the u1 fundamental shown in Figure 1 . Observation of the temperature dependence of the gas-phase spectral features and matrix isolation experimentsI2 have shown that some of the sharp Q branches are hot bands involving the CC bending vibration (vl0) and the v2 stretching vibration. The first and second overtones are also shown in Figure 3. With the three transitions obtained absorptions and eq 2, the harmonic frequency w2 = 2185 for f 12 cm-I and the anharmonicity constant X2, = -18 f 5 cm-' were obtained. It is estimated that the CEC u2 = 6 absorption should appear between 12400 and 12 500 cm-I. Figure 5 shows the absorption around the C-H u = 4 region in the range between 12 000 and 13 000 cm-I. The bands between 12 100 and 12 500 cm-l are negligible compared to the C-H u1 = 4 transition a t 12 684 cm-I, but an expansion (X30) of the region below 12 500 cm-' (see inset in Figure 5) shows at 12 332 cm-l an absorption that can be assigned to the combination band 4vl - 2v10. To the low-energy side the bands whose maximum are at 12232 and 12 130.3 cm-l are assigned as 4vl - v9 and 4vl - v5. To the high-energy side at 12415 cm-l there is a band that, based on
-
its position, cannot be assignable as a sum or difference band of the 4vl transition with any of the fundamentals of the molecule. This band at 12415 cm-' can be tentatively assigned as the C=C u2 = 6 absorption. With this assignment, the experimental point for u2 = 6 on a Birge-Sponer plot (Figure 6) (AE/u2 vs. u2) is on the straight line obtained for the C=L absorptions using the energies corresponding to the vibrational quantum levels u2 = 1, 2, and 3. Also the peak cross sections for CEC absorptions in Table I11 indicates that although there is a reduction of 3 orders of magnitude in the peak cross section in going from the fundamental to the first overtone, after that the generally observed trend of approximately 1 order of magnitude decrease per quantum level of excitation is observed in going from u2 = 2 to u2 = 3. If this trend continues for higher levels, as occurs with C-H cross sections (see Table 111), the expected cross section for the CGC (u2 = 6) absorption should be on the order of pm2/molecule. An estimated value of the cross section for the tentatively assigned C = L (u2 = 6) absorption was obtained by comparing the magnitude of this peak to the magnitude of the neighboring 4v1 absorpton. The resulting estimate for the 6 ~ cross 2 section is then 0.5 X pm2/molecule, in good agreement with prior expectation. Anharmonicities and Cross Sections. The value obtained for the C-H anharmonicity constant XI1= -49 f 2 cm-' is in good agreement with the one obtained in ref 6 using the fundamental and lower overtones (ul = 1, 2, 3) of CF3C=CH. It is also close to C-H anharmonicity constants of compounds such as C2H2and C2HD.18-21 Saturated compounds of the type X3CH show anharmonicity constants that typically are larger (-60 cm-1).14 The value X2, = -18 f 5 cm-I obtained for the anharmonicity constant of the e C stretching has not been previously reported. As a check on the above value, the stretching anharmonicity constant can also be approximately determined from the relation D = -W22/4x22. After substituting the values for D, the dissociation energy for the C W bond (80417 cm-1)22and w2 = 2185 crn-', we obtain X2, = 15 cm-', which is in good agreement with our experimental value. The cross sections for both C-H and C=C bonds have similar magnitudes for the fundamental transitions. They decrease by 2 orders of magnitude for the C-H stretch and 3 orders of magnitude for the CEC stretch in going from the fundamental (18) Scherer, G.; Lehmann, K. K.; Klemperer, W. J. Chem. Phys. 1983, 78, 2817. (19) Hayward, R. J.; Henry, B. R. Chem. Phys. 1976, 2.2, 387. (20) Baldacci, A.; Ghersetti, S.;Hurlock, S. C.;Narahari Rao, K. J . Mol. Spectrosc. 1976, 59, 116. (21) Allen, H. C., Jr.; Tidwell, E. D.; Plyler, E. K. J . Am. Chem. SOC. 1956, 78, 3034. (22) Weast, R. C., Ed.Handbook of Chemistry and Physics, 56th ed.; Chemical Rubber: Cleveland, OH, 1979.
3958
J . Phys. Chem. 1986, 90, 3958-3964
to the first overtone. For higher overtones, there is an approximate order of magnitude decrease for each quantum level of excitation. Line Widths. The simplest picture of overtone spectra would produce only one transition for each C-H upper state (ul = 0 v1 = 1, 2, 3, 4, 5, 6, ...). In reality one has additional absorption bands close to the main absorption. If there is a high density of vibrational and combination states and a substantial fraction of them interact resonantly, the vibrational spectrum will consist of broad overlapped bands in which the secondary transitions borrow some intensity from the main transition. The nature of the coupling between states will determine the overall appearance of the vibrational spectrum. For this particular molecule, CF3C= CH, the overtone absorptions involving the C-H stretch show only (at each side and separated from the main absorption) bands due to sum and difference transitions which originate from interactions with low-frequency modes, for example, the transitions found for (vu1 f vl0) and (vuI f v7). Also, the hot-band transition (u7 (v7, v v , ) ) which accompanies the main absorption (0 uul) increases its separation in energy from the main band as the quantum number of the upper level increases. The C-H vibrational overtone spectra obtained for CF3C=CH in the present investigation do not show resonant interactions in which the vibrational bands become broad due to superposition of many vibrational bands with slightly different band centers. An example of this would be saturated molecules like CF3H and (CF3),CH, where one lower molecular energy state is coupled with substantial line strength to several close-lying upper states. The bandwidths observed for C-H absorptions of C F 3 C ~ C H are relatively constant. Except for the full width a t half-maximum (fwhm) = 55 cm-’ observed for u1 = 5 , the fwhm for transitions to u1 = 2, 3, 4, and 6 are between 30 and 35 cm-I, which indicates there is no strong resonant couplings between these states and other vibrational fundamentals or overtones. Also, no apparent intensity enhancement of the combination bands due to resonance between the main absorption and the combination bands is observed. In general, the interactions that give rise to sum and difference bands (v7, u v I ) ) are and the interactions that produce hot bands (v7 very strong because these bands are observed for all levels studied. Although these couplings are strong, they are not resonant interactions in nature as is the case for saturated compounds. In compounds such as CF3H and (CF,)$H there is a Fermi resonance involving the C-H bending overtone and the C-H stretch. The main reason for the absence of this Fermi resonance in C F 3 G C H is the energy mismatch between the overtone of the bending mode and the C-H stretch.
-
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Conclusions The spectrum of C-H and CEC overtones of 3,3,3-trifluoropropyne has been investigated with intracavity photoacoustic spectroscopy and standard infrared techniques. Three types of vibrational transitions are identified for C-H absorptions of this molecule: (a) the main overtone transition (0 uv,), (b) the hot band (v7 (u7 uv,)), and (c) combination bands of the type (vuI f vIo) and (uv, f u7). The hot-band absorptions v, (v7 uvI) observed for all the transitions from u = 1 to u = 6 show strong interaction between the C-H stretching mode ( u l ) and the C-H bending mode ( v . ~ ) . The same interaction between vibrational motions is also identified for other molecules with single acetylenic C-H stretching vibrations such as NECH and (CF3)3CC=CH. This strong coupling is nonresonant in nature. Combination bands such as vlvl f vl0 or ulvl f v7 are close in energy to the main absorption but they do not seem to be enhanced by resonance with it. The structure on top of the overtone absorptions u1 = 2 to v1 = 6 is probably due to a set of hot-band sequences that involve the main C-H transitions ( u l ) and hot bands of the CCEC bending transition (vl0). The bandwidths (fwhm) for all main overtone absorptions are approximately 30-35 cm-’, indicating that there are no new levels or combination states that exhibit a strong resonant interaction with this absorption at different levels of excitation. Fundamental bands due to C=C and C-H transitions have cross sections that are comparable. For overtones, there is a considerable reduction in the cross sections of CEC transitions compared with C-H cross sections. The band shape of the acetylenic stretching (vz) does not show a simple PQR structure. The shape of the band is mainly due to hot bands involving the C C S bending vibration (vl0) and the (v2) stretching vibration. The calculated anharmonicity constant (X2J is in agreement with an estimated value from the dissociation energy D and the harmonic frequency. 3,3,3-Trifluoropropyne represents an interesting example of isolated C-H absorptions where overtone transitions are not obscured by the presence of Fermi resonances between the main absorption and combination states.
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Acknowledgment. We thank the National Science Foundation for support of this work under CHE82-06976 and CHE85-06957. We also thank Professor Mark Ratner for some useful discussions. Registry No. CF3C=CH, 661-54-1.
Spectroscopic Evidence for Spatial Correlations of Hydrogen Bonds in Liquid Water J. L. Green, A. R. Lacey, and M. G. Sceats* Department of Physical Chemistry, University of Sydney, N.S.W., 2006, Australia (Received: February 3, 1986)
The low-frequency shoulder of the OH stretching Raman spectrum is developed as a probe of in-phase collective motions in liquid water. Its relative intensity approaches that of ice I as the supercooled liquid temperature tends toward the conjectured thermodynamic singularity in the vicinity of -46 O C . The collective band appears despite the large disorder of the OH stretching frequencies in the liquid compared to the strength of the resonance coupling. The resonance condition required for collective OH motions leads us to conjecture that patches of water molecules with similar hydrogen bond energies, which are capable of sustaining the resonance, appear as water is supercooled toward T,.
Introduction Coupled energy and density fluctuations appear as liquid water is supercooled.’*2The conjectured transportbs ( 1 ) Angell, C. A. In Wafer-A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1983; Vol. 7, Chapter 1. (2) Angell, C. A. Annu. Rev. Phys. Chem. 1983, 34, 593.
and X-ray scattering9 divergences have led to an explanation of many of the observed anomalous properties of water as well as R: .I.Angell, ; C. A. J. Chem. Phys. 1976, 65, 851. (4) Zhelenzny, B. V. R u s . J. Phys. Chem. (Engl. Transl.) 1969,43, 131 I; 1968, 42, 950. (5) Angell, C.A.; Sichina, W. J.; Oguni, M. J . Phys. Chem. 1982,86, 992. (3) Speedy,
0022-3654/86/2090-3958$01.50/0 0 1986 American Chemical Society
