3027
J . Phys. Chem. 1991,95, 3027-3031 excitation profiles, but this deviation from normal expectations may help establish the shape of the potential.
Acknowledgment. Our thanks are due the Natural Sciences and Engineering Research Council of Canada and the Ministbe
de I’Education du QuCbec for financial assistance. We are also indebted to Dr.H. Le Thanh for the synthesis of the Schiff base and to Dr.R. Savoie and Dr.M. PCzolet for permission to use their Raman spectrometer. We also thank Marie Pigeon for her assistance in performing Raman measurements.
Vibrational Anaiysls of C-H Stretching Overtones in Pyridine and 2,6-Lutidine Roberto Bini, Paolo Foggi,* LENS (European Laboratory for Non-Linear Spectroscopy), Largo E. Fermi 2, I-50125 Firenze, Italy, and Dipartimento di Chimica, Universitci di Firenze, Via Gin0 Capponi 9, I-501 21 Firenze, Italy
and Raffaele Cuido Della Valle Dipartimento di Chimica Fisica, Universitci di Venezia, Calle Larga S. Marta, Dorsoduro 21 37, 301 23 Venezia, Italy (Received: April 24, 1990; In Final Form: October I , 1990)
Aryl and methyl C-H stretchings of pyridine and 2,6-lutidine in the liquid phase have been studied in the region of the fundamentals and of the overtones up to the fifth. Standard spectrometers were used to investigate IR and near-IR regions. Thermal lensing laser spectroscopy was used in the visible region. An = 1 , 2 spectra show a complex pattern that becomes simpler on going to higher An values. In this case, the interpretation of the spectra can be made by using the local-mode approach. Stretching frequencies are mainly perturbed by the local environment of bonds, and then new details on molecular structures can be obtained. The observation of the double bands in high-overtone spectra of pyridine is due to differences between properties of the C-H bonds in the 2,6- and 3,4,5-positions. The lower frequency structures in 2,6-lutidineare attributed to conformationally nonequivalent C-H bonds of the methyl groups. Nonnegligible interactions between methyl C-H bonds and the nitrogen atom are responsible for a smaller anharmonicity of the in-plane C-H stretching.
Introduction The study of vibrational overtones is an interesting subject in both experimental and theoretical fields. It has been found that properties of high overtones of X-H stretching are mainly due to the local environment of the bonds.I4 The analysis of overtone spectra can then provide further details of molecular structures and of vibrational potential surfaces. In addition, the application of new laser techniques5-* has made high-overtone investigation a reliable spectroscopic tool. The local-mode description’-4.6+12 seems to be the most convenient model for the interpretation of the experimental data mainly for X-H stretching overtones. In this model, the X-H bonds behave as highly anharmonic weakly coupled oscillators. At low energies, the coupling between these nearly resonant oscillators is effective, and the familiar normal-mode description, in terms of delocalized harmonic oscillators, is a good approximation. At higher energies, the bonds are detuned from resonance, and the local-mode description, in terms of isolated anharmonic oscillators, becomes more appropriate. Localization is due to the quenching of the interbond coupling by the bond anharmonicity a t high energies, and it is not necessarily related to a bond-separable potential surface. Delocalized modes, Le. those involving the motions of two or more bonds, are still present at high energies. Purely local modes, the stretching of single bonds, are more anharmonic than delocalized ones. The transition dipole moment is higher for localized modes$*” and consequently they dominate the high-overtone spectra.4J4Js Absorption spectroscopy thus favors the observation of modes with a prevalent local character. However, it is possible to observe in the high-frequency region more delocalized modes with the help of other spectroscopic techniques.I6 It has recently been shown that, if all of the relevant interactions are included, the normal-mode and local-mode treatments become mathematically equi~alent.”-’~For the present work, dedicated to the properties of individual aryl and methyl C-H bonds at high overtones, the purely local-mode pictureZZois sufficient; as pointed To whom correspondence should be addressed.
out in previous ~ o r k s , ~all l -interbond ~~ interactions may be neglected. This is not in principle a good approximation, especially for methyl C-H bonds. However, it is possible to use it when local environment perturbations are larger than interbond coupling terms. The bonds are modeled as Morse oscillators, which may be treated analytically and appear to describe very well the C-H bonds. In this approximation, the frequency of a transition from the ground to the nth excited state of a given bond is simply given by the Morse r e l a t i ~ n : ~ - ’ ~ ? ’ ~ wgn = wSn x p ( n 1) (1)
+
+
(1) Hayward, R. J.; Henry, B. R. J . Mol. Spectrosc. 1975, 57, 221. (2) Mortensen, 0. S.; Henry, B. R.;Mohammadi, M. A. J . Chem. Phys. 1981, 75,4800. (3) Child, M.S.; Lawton, R. T. Faraday Discuss. Chem. SOC.1981, No. 71, 273. ( 4 ) Child, M. S.; Halonen, L. Adu. Chem. Phys. 1984, 57, 1. (5) Long, M. E.; Swofford, R.L.; Albrecht, A. C. Science 1976, 191, 183. (6) Swofford, R. L.; Long, M. E.; Albrecht, A. C. J . Chem. Phys. 1976, 65, 179. (7) Patel, C. K. N.; Tam, A. C.; Kerl, R. J. J. Chem. Phys. 1979,71, 1470. ( 8 ) Kliger, D. S . Acc. Chem. Res. 1980, 13, 129. (9) Stannard, P. R.; Elert, M. L.; Gelbart, W. M. J . Chem. Phys. 1981. 74, 6050. (10) Watson, I. A.; Henry, B. R.; Ross,I. G. Spectrochim. Acta 1981,37A, 857. (11) Halonen. L.; Child, M. S. Mol. Phys. 1982, 46, 239. Halonen, L. Chem. Phys. Lett. 1982,87, 221. (12) Mills, I. M.; Robiette, A. G.Mol. Phys. 1985, 56, 743. (13) Wong, J. S.;Moore, C. B. J. Chem. Phys. 1982. 77, 603. (14) Perry, J. W.; Zewail, A. H. J . Chem. Phys. 1979, 70, 582. (15) Burberry, M. S.; Morrell, R. J.; Albrecht, A. C.; Swofford, R.C. J. Chem. Phys. 1979, 70, 5522. (16) Abbate, S.; Longhi, G.; Ricard, L.; Bertucci, C.; Rwini, C.; Salvadori. P.; Moscowitz, A. J. Am. Chem. Soc. 1989, I l l , 836. (17) Lehmann, K. K. J . Chem. Phys. 1983, 79, 1098. (18) Lehmann, K. K. J . Chem. Phys. 1986,84, 6524. (19) Della Valle, R. G.Mol. Phys. 1988, 63, 611. (20) Moller, H. S.; Mortensen, 0. S. Chem. Phys. Leu. 1979, 66, 539. (21) Greenlay, W. R. A.; Henry, B. R. J . Chem. Phys. 1978, 69, 82. (22) Henry, B. R.; Mohammadi, M. A.; Thomson, J. A. J . Chem. Phys. 1981, 75, 3165. (23) Cough, K. M.; Henry, B. R. J . Phys. Chem. 1984,88, 1298.
0022-3654/91/2095-3027%02.50/0 0 1991 American Chemical Society
3028 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991
Bini et al. 3100 in
0
in
m
2
-€
2903
C
Q
2800
. ...
2700
I
26OC 2600
31 00
o I A n (cm -1) Figure 1. Spectra of liquid pyridine (lines) and 2,6-lutidine (dots) in the An = 1-6 regions. Frequency units have been scaled in the form w/An, while intensities have been normalized to the same maximum.
where obis the stretching frequency and xg is the anharmonicity of the bond. Both parameters are sensitive to the environment experienced by the C-H bond. Experimental and Data Handling Section Pyridine and 2,6-lutidine from Merck (UV grade compounds) were purified by distillation over NaOH and then sealed in quartz cells under dry nitrogen. The spectra of the liquids were recorded at room temperature. For the An = 1 (fundamentals) region, a 0.05" KBr cell was used in a Perkin-Elmer 225 spectrometer. To investigate the An = 2 spectra, a I-mm quartz cell was used in a Cary 14 spectrometer with the near-IR light source. The An = 3-5 overtones were measured in the same instrument with a IO-" quartz cell. The An = 5, 6 overtone spectra were recorded by using thermal lensing detection5**with a 10-mm quartz cell. The experimental apparatus is described elsewhere.24 The overlapping between spectra of An = 5 overtones measured with both methods was good and was used to calibrate the intensity response of our thermal lensing setup. The spectra of the fundamentals (An = 1) and all recorded overtones (An = 2-6) are shown in Figure 1 . All spectra were normalized to the same maximum intensity, and frequency units were scaled in the form w / A n , in order to allow a direct comparison among the various overtone band shapes. Where necessary, air wavelengths were converted to vacuum frequencies (cm-I). The spectra of pyridine are similar to previous data.25.26 In order to extract frequency values that are as precise as possible from the broad spectra recorded, all data were digitized and analyzed through a computer-assisted "peak pealing" procedure in terms of Gaussian bands superimposed on a linear background. Marquardt's algorithm2' was used to solve the nonlinear least-squares problem. The best experimental frequency estimates for pyridine and 2,6-lutidine are collected in Table I. Due to the variety of instruments and spectral settings used, the measurement errors for the various overtone manifolds differ widely. A separate determination of the statistical errors (standard deviation 6, of frequency estimates) for each manifold yielded 6, = 0.9, 1.3, 2.2, 3.0, 4.0, and 5.2 cm-I for An = 1-6. The frequencies estimated by using alternative band shapes (Gaussian, Lorentzian, or pseudo-Voigt profiles) were within the statistical (24) Bini, R.;Foggi, P.; Della Valle, R. G . J. Mol. Srnrcr. 1990,218, 117. (25) Mizugai, Y.; Katayama, M. Chem. Phys. Leu. 1980, 73, 240. (26) Avouris, Ph.; Demuth, J. E. J . Chem. Phys. 1981, 75, 5953. (27) Marquardt, D.W. J. Sac. Ind. Appl. Marh. 1963, 1 1 , 431.
k I
I
I
1
I
I
1
2
3
4
5
6
A n
Figure 2. Plots of Morse progressions for 2,64utidine. Experimental frequencies (solid symbols) are shown as w / A n vs An. Straight lines are pure-local-mode fits, neglecting all interbond interactions, for C-H,,,, C-Hh, and C-H,,. Open symbols represent frequencies which have not been included in the fit. 31 00
3000
2900
€
v
. C
Q
3 2800
27OC 4
260(
I
1
I
I
I
I
1
2
3
4
5
6
A n
Figure 3. Plots of Morse progressions for pyridine. The overtones for C(3,5)-H and C(4)-H are not resolved and are labeled 3,4,5. Symbols are as in Figure 2, and straight lines are pure-local-mode fits.
error. Systematic errors, due to limitations of the instruments, were 1 (An = l), 15 (An = 2-4), and 10 (An = 5,6) cm-I. These errors were the main source of indeterminacy in the spectroscopic parameters of the C-H bonds. Spectral Interpretation The experimental frequencies lie quite close to sets of Morse progressions (Figures 2 and 3). The progressions may be identified by starting from the higher overtones and extrapolating down toward the fundamentals. In this way, it is clear that for 2,6lutidine the An = 3 band is the superposition of two unresolved peaks. The Morse progressions (eq 1) are more perturbed on going toward the fundamentals, where the interbond couplings become active, effectively delocalizing the vibrational excitations. The phenomenon of localization is shown very clearly by the spectral
C-H Stretching Overtones in Pyridine and 2,6-Lutidine
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3029
TABLE I: Experimental and Best-Fit Frequencies (cm-') for f6-Lutidinc and PyridW 2,6-lutidine pyridine mode An fit exp fit exp wnwl(methyl) 1 2 904 (2926, 2956)
TABLE II: h l - M o d e Bond Frequency o,A n h a d c i t y x , and Parameters of tbe Morse Potential from the Least-Squares Fir
2 3 4 5 6
5690 8366 10925 13368 15696
(5728) 8369 10909 13392 15686
d,(methyl)
1 2 3 4 5 6
2861 (2857) 5653 (5651) 8377 8369 11031 11042 13617 13612 16133 16134
aryl)
1 2 3 4
'
5
~o"~-~(aryl) 1
3031
2 3 4
5950 8758 11454 14038 16510
5 6
pyridine
(2988, 3026, 3066) (5950) 8751 11450 14056 16501
3037 5961 8773 11473 14060 16535
(3031, 3056, 3080) (5907, 5956) 8785 11474 14037 16548
Combinations WIWl+
w'2.6
w12.6 wl3-5 WZin
@lout
+ w12.6 + w13-5
+ wl3-5 + wIw,
5 808 (5776, 5887) 6062 (6092) 8554 (8577)
x , cm-l C-H bond w , cm-l 2930 f 12 -34 f 2 in Out 3-5 2,6 3-5
3019f 3143 f 3105 f 3149f
17 12 14 14
-58i3 -56i2 -56f 2 -56f 2
D, kcal/mol
a,
178 113 126 122 126
A-'
1.43 I .86 1.75 1.75 1.75
"The errors shown are estimates of the standard deviations u.
2993 (3004) 5 872 (5835, 5867) 8639 8651 11294 11296 13835 13810 16264 16277
6
molecule 2.6-lutidine
5985 (5992) 6029 (6034) 6073 (6102,6132)
"The experimental frequencies in parentheses have not been included in the fit and are shown for convenience near similar computed frequencies. In the combinationssection, the notation w,I + wbl means that two quanta are distributed so that only one quantum is in each oscillator. Accordingly, 02 + ubi means that two quanta are in oscillator a and one quantum is in oscillator b. band shapes (Figure l), which are quite complex for An = 1 and 2 and are reduced to sets of broad structureless bands at higher overtones. The strongly coupled modes at low An, normal in character, become progressively decoupled at high An, where they are essentially reduced to the motion of isolated C-H bonds. The bonds involved may be identified directly by comparing the spectra of pyridine and 2,dlutidine (Table I; Figures 1-3). Pyridine overtone spectra have only two bands for An = 3-6, but 2,6-lutidine spectra show a single band in the same regions and an additional structure at lower frequencies. The higher progression, common to both molecules, may thus be assigned to the aromatic C-H bonds in the 3,4,5-positions (C(3)-H, C(4)-H, and C(5)-H). Intermolecular interactions in the liquid broaden the bands and obscure the small splitting that must be present due to the slight difference between the properties of the C(3,5)-H and C(4)-H bonds. The lowest frequency progression in pyridine, which has no equivalent in 2,64utidine, is assigned to the C(2)-H and C(6)-H bonds. The two low-frequency progressions present only in 2,6-lutidine are due to stretchings of nonequivalent methyl C-H bonds. The nonequivalence of the environment experienced by the methyl protons has often been demonstrated in stretching overtone spectra in molecules like methyl-substituted pentanes,22toluene and the xylenes,23durene,28 a ~ e t a l d e h y d eand , ~ ~transition-metal complexes.30 As in the previous cases, the methyl groups in ~
(28) Perry, J. W.; Zewail, A. H.J . Phys. Chem. 1982, 86, 5197. (29) Findsen, L.A.; Fang, H.L.;Swofford, R.L.; Birge, R.R.J . Chem. Phys. 1986,84, 16. (30) Henry, B. R.;Goswami, P. C.; Swanton, D. J. Chem. Phys. Lett. 1988, 144, 527.
2,6-lutidine can be considered nearly free rotors. The height of the 6-fold barrier in the isolated molecule has been estimated to be about 0.39 kcal/mol in a recent microwave e~periment.~' Under these conditions, the nonequivalence of the methyl protons may only be detected with techniques having a short effective time scale, like the optical spectroscopies. In fact, the frequencies involved in the experiments are a hundred times higher than that of the CH3 rotational motion. The protons appear identical in NMR and microwave experiments, which have time scales long with respect to the rotation time of the methyl groups. The barrier opposing the CH3 rotation in 2,6-lutidine is higher than that in toluene (0.014 kcal/molz3), as a consequence of the interactions between the hydrogens and the nitrogen. The lowest energy conformer for 2,6-lutidine is likely to have C, symmetry, with the methyl groups in an eclipsed configuration, i.e. with one C-H bond in the ring plane (C-Hi,), as in toluene and the xyle n e ~ .A~staggered ~ configuration, with one C-H bond nearly perpendicular to the ring plane, would exhibit three nonequivalent bonds, rather than two. Our spectra show only two progressions and confirm, within their resolution, the hypothesis of eclipsed geometry. One of the two progressions is due to the two C-H bonds in the ring plane (C-Hi, bonds), while the other is due to the four C-H bonds a t 60' with respect to the plane (C-H,, bonds). The slope of one of the C-H progressions, and thus the anharmonicity of the corresponding bond, differs widely from that of other bonds in 2,6-lutidine and pyridine. The steeper progression, being split into two fundamental components, can be assigned to the out-of-plane C-H,, bonds, while the other progression is due to the in-plane C-Hi, bonds. An extremely perturbed bond must be responsible for this progression (see next section). Computed Frequencies
In the pure local-mode approximation, all interactions between different C-H bonds are neglected and eq 1 is assumed to hold! Equation 1 is the eigenvalue equation of the Morse Hamiltonian, which represents the stretching of a single bond with respect to the equilibrium bond length re:4,32
H = -(h2/2p)d2/d9
- D[l- e-a(rre)]2
(2)
Here p is the appropriate mass for the oscillator, D is the dissociation energy of the bond, and a-I is a measure of the "width" of the potential well. The parameters D and a are related to the frequency and anharmonicity parameters w and x by D = _w2/4x and a = ( 2 p ~ / h I ' / ~ . These parameters are sensitive to the environment experienced by the bond. The presence of a single band at high overtones for both C(3,4)-H and C(5)-H bonds implies that these bonds may be treated as approximately equivalent. The pure-local-mode approximation (eq 1) is expected to break down badly in the regions of the fundamentals (An = 1) and of the first overtones (An = 2), where strong interactions are present (Figures 1-3). We have included in the fit only the experimental data for An = 3-6 and extrapolated the results down to An = 1. The best parameter estimates from the least-squares fit are in Table 11. The extrapolated frequencies for the fundamentals, first overtones, and combinations are reported in Table I. A safe assignment ~
(31) Caminati, W.; Di Bernardo, S. Chem. Phys. Lett. 1990, 171, 39. (32) Nieto, M. M.; Simmons, L. M. Phys. Reu. A 1979, 19,438.
3030 The Journal of Physical Chemistry, Vol. 95, No. 8,1991 cannot be made for these modes; thus, the proposed correspondence between experimental and computed frequencies is rather arbitrary. The Morse parameters for the C(3,5)-H and C(4)-H bonds of pyridine and 2,6-lutidine are essentially identical, within our resolution, and are also consistent with the parameters for C-H bonds in benzene (w 3150 cm-', x -60 ~ m - ' ) other , ~ ~ aromatic compounds,23and alkanes.'-2'-22 The renormalized harmonic frequencies of the fundamentals, w B 1 ,are directly related to the parameters of the Morse prog r e s s i o n ~as , ~wB1 ~ = wB + 2xB(eq 1 for n = 1). For pyridine, with a reduced mass p = ( m H m C ) / ( m H mc) for each oscillator, in agreement with the concept of a bond-localized oscillator, the renormalized frequencies correspond to force constants K2, K6 = 4.91 f 0.06 and K3,K5 K4 = 5.05 f 0.06 mdyn/8,. These values should be compared with those obtained from a purely harmonic force field ~ p t i m i z a t i o n :K~2~, K = 5.027 8 13, K3, K5 = 5.135426, and K4 = 5.104510 mdyn/i. Our data are just outside one standard deviation u from the harmonic data, which are expected to be very precise. Furthermore, the difference between K3 or K5 and K4 is so small, with respect to our resolution, that the equivalence of the C(3,5)-H and C(4)-H bonds is justi fied. From the diagram in Figure 2, it is apparent that the C-Hi, bonds are weaker and less anharmonic than the relatively unperturbed C-H,,, bonds. The slope of the C-Hi, progression is smaller than that of the C-H,, progression, one that runs parallel to the aryl C-H bond progression. The dissociation energy depends on both the bond strength and the anharmonicity and may be estimated (Table 11) as D -w2/4x by assuming that the C-H bonds continue to behave as Morse oscillators all the way up to the dissociation limit.4 A more accurate evaluation of the dissociation energy D can be obtained by using a LippincottSchroeder p ~ t e n t i a l . ~Following ~ . ~ ~ the procedure shown in ref 22, we estimate that the C-Hi, dissociation energy is 30% smaller than that obtained by using the Morse expression. The C-H,,, dissociation energy is only 20% smaller; however, the C-Hi, dissociation limit is still 40% higher than that of C-H,,,. Our experimental results suggest that the dissociation energy for the C-Hi, bond is much higher than for the C-H,,, bonds, as its reduced bond strength is more than compensated by the reduced anharmonicity. A reduction of anharmonicity has been previously observed in methyl-substituted alkane^.^^^^^ This phenomenon was correlated to solvent viscosity and steric crowding. A viscosity-dependent barrier, more active at high-amplitude vibrational motions, causes the local methyl C-H stretching potential to be less anharmonic.22 In our case, a higher dissociation energy of the C-Hi,, bond is to be expected if the lowest energy conformer has the in-plane hydrogen pointing toward the nitrogen, rather than in the opposite direction. With this geometry, the equilibrium distance Hi,.-N may be estimated at about 2.6 8,. This distance is significantly short, thus suggesting the possibility of nonnegligible N-H interactions. The average C-H bond length in an excited state, (nlrln), grows with the quantum number n. Consequently, the N-H distance is further reduced at high n, the N--H interactions become more repulsive, and the dissociation barrier is raised. This effect is quite significant, as the average displacement of Hi,from the potential minimum, (nlr - r#), is 0.5 8, for n = 6. This value was obtained by numerical integration in terms of Morse oscillator wave functions.32 A useful approximation, valid for large values of the anharmonicity ratio k = Iw/xl, k >> n, is
-
-
+
-
-
(nlr - r,ln)
-
3[(n
+ 1/2)/kl/a
(3)
This approximation is obtained from the exact expre~sion,~ by using the asymtotic expansion of the di-r function3' and discarding (33) Page, R. H.; Shen, R.Y.;Lee, Y. T. J . Chem. Phys. 1988,88,4621. (34) Golab, J. T.; Sprague. J. R.;Carron, K.T.; Schatz, G.C.; Van Duyne, R. P. J . Chem. Phys. 1988,88,1942. (35) Lippincott, E. R.; Schroeder, R. J . Chem. Phys. 1955, 23, 1131. (36) Henry, B. R.; Miller, R. J. Chem. Phys. Lett. 1979, 60, 81.
Bini et al. 0
Lutidine
(3,4,5)
I
-
aJ a
2 1M
3
0-
; -1-
-tl5
-2-
-3
2
4
Figure 4. Experimental (symbols) and calculated (curve) peak absorption
coefficient (cm-') as a function of An. terms of order (n/k)2. The error involved in eq 3 is small, 9% for n = 6, and the linear dependence of ( r - r e ) on n remains approximately correct up to the dissociation limit. Absorption Intensities
The absorption intensities, q(O l & ~ ) l ~ , depend on various constant factors that are incorporated in the parameter C and on the relationship between the dipole moment of the bond and the displacement of the hydrogen nuclei. The simplest p o ~ s i b i l i t y ~ ~ ~ ~ is p ( r ) a r, which may be seen as the first term of an expansion in powers of the displacement coordinate r.9 The matrix element (OIpln), for large values of the anharmonicity ratio k = Iw/xI, may be approximated as4 (Olpln)
-
(O(pll)[(k1-"n!)/n2]'/2
(4)
The best fit to the experimental peak absorption coefficients for the aryl C-H bonds, using eq 4 for the matrix elements, is obtained for q(Olpll)12 = 848 cm-I and k = 27.5. The fit is excellent (Figure 4) and reproduces the observed variation of the absorption intensity over more than 5 orders of magnitude. However, the value of k required to fit the absorption intensities is smaller then the value of k = Iw/xl 56 obtained for the aryl C-H bonds from the frequency data. The absorption intensities thus decay with n more slowly than predicted by the potential anharmonicity alone. Electrical anharmonicity, i.e. a nonlinear relationship between the dipole moment and the nuclear displacement, must therefore supplement the mechanical anharmonicity. Several nonlinear forms of the dipole moment have been tested with good r e s ~ l t s . ~ In * general, ~ ~ * ~ it~ has ~ ~ been ~ shown4 that the electrical anharmonicity increases the absorption intensities if the maximum of the dipole function p ( r ) for stretchings is a t values of r that are larger than the equilibrium length re.
-
Conclusions
Our work confirms the potential utility of C-H stretching overtone spectroscopy as a means of chemical labeling. It shows that C-H overtones act as a kind of "infrared chromophore", as suggested by Amrein et A careful analysis of the spectral parameters provides further information on the properties of individual bonds. Working in the optical frequency region, it is possible to observe different conformations not accessible with other spectroscopic techniques. Charge separation, due to the higher electronegativity of nitrogen with respect to carbon, is well-known for pyridine. The charges on the C(2,6)-H bonds and, to a lesser extent, on the C(4)-H bond, are reduced with respect to those on the C(3,5)-H bonds. The observed order of the bond strengths, K2,K6 < K4 (37) Abramowitz, M.; Stegun, I . A. Handbook of Mathematical FuncDover: New York, 1965. (38) Amrein, A.; Dubal, H. R.; Lewerenz, M . ; Quack, M. Chem. Phys. Lett. 1984, 112, 387. (39) Lewerenz, M.; Quack, M. Chem. Phys. Lett. 1986, 123, 197.
tions:
3031
J. Phys. Chem. 1991,95, 3031-3037
-
K3, Ks,is thus to be expected on chemical grounds. For 2,6-lutidine, our results suggest that the more stable conformer has a C , symmetry configuration, with one of the methyl C-H bonds pointing toward the nitrogen atom. Ab initio calculations to check this hypothesis are in progress.
Acknowledgment. This work was supported by the Italian Minister0 dell’Universitl della Ricerca Scientifica e Tecnologica (MURST) and Consiglio Nazionale delle Ricerche (CNR). We thank Enzo Stefan0 for having provided the peak analysis program and Vittorio Lucchini for useful suggestions.
Vibrational Spectra, Structures, and Normal-Coordinate Analysis of AI-COP Complexes Isolated in Solid Argon A. M. Le Quire, C. Xu, and L. Manceron* Laboratoire de Spectrochimie Moliculaire (CNRS UA 508), Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France (Received: May 29, 1990: In Final Form: October 4 , 1990)
The codeposition of atomic AI and carbon dioxide molecules in argon matrices led to the formation of AIC02 molecules, which are found to reversibly interconvert between two geometrical isomers. The low-temperature form presents a C, symmetry, with a large inequivalenceof the two CO bonds. The higher temperature form has a ring structure in which the metal interacts symmetrically with the two oxygen atoms. Normal-coordinate analysis based on four isotopic precursors (12C1602, 13C160 2. 12C1802, and 1 ~ 1 6 0and 1 8a0harmonic ) model enable a determination of some molecular constants. The C, symmetry structure has CO bonds with force constants almost corresponding to those of double and single bonds (FC+ = 13.1 and Fc4 = 6.75 mdyn A-I) and a relatively strong A 1 4 bond (FAM= 2.2 mdyn A-’), while the C, symmetry structure has equally rturbed CO bonds comparable in stiffness to carbonate species and also two looser A 1 4 interactions (FA14= 1 mdyn The OCO valence bond angles are estimated in the 120 f 5’ and 115 f 5’ ranges, respectively. Temperature studies of the relative population yield an enthalpy difference of 1.55 0.4 kJ/mol between the two isomeric forms. For larger AI clusters, reductive elimination is evidenced upon warming the sample above 30 K, yielding A120 and, presumably, CO.
fi).
*
Introduction Fixation of carbon dioxide by metal centers is a possible route to the chemical activation of this readily available molecule.’ On the other hand, the spectroscopy of well-defined, “simple” coordination compounds of C 0 2can serve as spectral reference points for comparison with surface specie^.^^^ At the molecular cluster scale, in addition to the studies devoted to the C 0 2 complex formation on transition-metal center^,^ the complexation and activation by alkali metals have been thoroughly studied5-” in cryogenic matrices, but less attention was paid to other main-group metal-C02 complexes, in particular group 111 metals, which are very reactive with respect to C O bonds. In the case of aluminum the reaction AI C02 AIO* CO (1)
+
+
+
has recently been the subject of three different investigations in the gas phase,I2-l4 owing to the facile detection of either the AI atoms by fluorescence or the A10 product through intense chemiluminescence. A first studyI2 using a high-temperature fast-flow reactor monitored the A1 atom concentration in eq 1 over
the 300-1880 K temperature range. The authors deduced an activation energy of the order of 2.5 kcal/mol and an A 1 4 dissociation energy D 1 122 kcal/mol. They also conjectured that the non-Arrhenius behavior observed at high temperature (>750 K) could be due to a second reaction channel involving vibrationally excited C 0 2molecules with one or several quanta of v2 vibration (bending mode). More recently, a second studyI3 involved a crossed pulsed supersonic beam technique that allows scanning of the translational energy of the ground-state reagents. The A10* product is detected by chemiluminescence, and the authors noted a translational energy threshold t = 0.17 eV (3.9 kcal/mol) and a dissociation energy DA14 5.26 eV. An even more recent gas-flow study14 involved AI atom generation from trimethylaluminum photolysis in a higher pressure C02/AI buffer gas mixture and combined the two detection methods used in the other works. From kinetic studies, Parnis et aI.l4deduced that a simple extraction channel could not account for their data. A more complex reaction scheme of the type
=
AI
+ COP f
AICO:
~~~~~
( I ) Walther, D. Coord. Chem. Reo. 1987, 79, 135. (2) Bartos, B.; Freund, H. J.; Kuhlenbeck, H.; Neumann, M.; Lindner, H.; MiIller. K. Surf. Sci. 1987. 179. 59. (3) Wohlra6, S.;Ehrlich, D.;’Wambach, J.; Kuhlenbeck, H.; Freund, H. J. Surf. Sci. 1989, 220, 243. (4) Mascetti, J.; Tranquille, M. J . Phys. Chem. 1988, 92, 2177. (5) Jacox, M. E.;Milligan, D. E. Chem. Phys. Lerr. 1974, 28, 163. (6) ShaD. J. H.: Svmons. M. C. R. J . Chem. Sot. A 1970.3075. (7) Borel, J. P.; F k , F.: Pittet, A. J . Chem. Phys. 1981,74, 2120. (8) Kafafi, 2.H.; Hauge. R. H.; Billups, W. E.; Margrave, J. L. J . Am. Chem. SOC.1983, 105, 3886. (9) Kafafi, 2.H.;Hauge, R. H.; Billups, W. E.; Margrave, J . L. Inorg. Chem. 1984, 23, 177. (IO) Teghil, R.; Janis, B.; Bencivenni, L. Inorg. Chim. Acfa 1984.88, 115. (11) Manceron, L.; Loutellier, A.; Perchard, J. P. J . Mol. Srrucr. 1985, 129, 1 IS. (12) Fontijn, A.; Felder, W. J . Chem. Phys. 1977, 67, 1561. (13) Costes, M.; Naulin, C.; Dorthe, G.; Vaucamps, C.; Nouchi, G. Faraday Discuss. Chem. SOC.1987, 84, 75. (14) Pamis. J. M.; Mitchell, S. A.; Hackett, P. A. Chem. Phys. Lett 1988, 151, 485.
-
AI0
+
COP
A m 2
was needed, postulating the existence of an AICOz complex as a stable energy subminimum of the system. They found that abstraction and complexation channels have comparable magnitudes in the medium-pressure range of the work of ref 13 for the removal of AI and estimated a 9 kcal/mol lower limit for the AI-C02 binding energy. The knowledge of the vibrational spectrum of this complex would be useful to estimate the partition function and therefore the activation energy for complex formation. We report here an IR study of the different AI + C02 reaction species isolated in solid argon. Experimental Section The cryogenic refrigeration system, vacuum vessel, and experimental technique have been described elsewhere.ls High-
0022-3654/91/2095-3031$02.50/00 1991 American Chemical Society
