2486
J . Phys. Chem. 1992, 96, 2486-2490
titanium as compared to vanadium. With this in mind, the intrinsic bond strengths of TiV and V2 are expected to be similar, with V, somewhat more strongly bound. This is indeed found to be the case, with the two molecules having intrinsic bond strengths of 3.13 eV (TiV) and 3.25 eV (V?). In this case, the great difference in measured bond strengths for the two molecules (0.68 eV) results primarily from the increased promotion energy which is required to prepare the titanium atom for bonding (0.8 1 eV, as compared to 0.25 eV for vanadium). The remaining species listed in Table I (Ni,, NiPt, and Pt,) are all late-transition-metal diatomics for which the nd orbitals are quite contracted. In Ni, this contraction is so severe that 3d contributions to the chemical bond are essentially absent. In Pt,, however, relativistic contractions of the ns orbitals lead to better shielding of the 5d orbitals from the nuclear charge, causing the 5d orbitals to expand and become more accessible for chemical bonding. As a result, the intrinsic bond strength of Pt, is 0.85 eV greater than that of its coinage group congener, Au,. This implies a very strong interaction between the 5d orbitals on platinum, and suggests s~~du:d?~~d6:d6:~d?r;4 as the primary electronic configuration of fit,, giving a net bond order of 2 for the Pt, molecule. The NiPt molecule falls midway between Ni, and Pt2in its bond strength, undoubtedly because the combination of a very small 3d orbital on nickel with a large, accessible 5d orbital on Pt gives a 3d-5d bond intermediate in strength between those found in Ni, and Pt,.
strengths. An argument for the requirements needed to determine the bond strength by the onset of p r e d i i a t i o n has been presented, yielding two criteria which must be fulfilled for the method to be suocessful. First, the molecule must possess a very large density of electronic states at its lowest dissociation limit. Second, the lowest dissociation limit must generate repulsive electronic states since predissociation in a set of nested potential energy curves may not be efficient. The chemical bonding in TiV, V,, TiCo, VNi, Ni,, NiPt, and Pt, has been discussed in relation to the electronic configurations of the ground-state molecules (in so far as they are known). The contribution of the d orbitals to the measured bond strength has also been considered by taking into account the promotion energy required to prepare the atoms for bonding and by comparison with the filled d-subshell coinage metal diatomics. The d-orbital contributions to the chemical bond in TiV, V2, Ni2, NiPt, and Pt, are found to be 1.10, 1.22,0.04, 0.46, and 0.85 eV, respectively. In the cases of TiCo and VNi it is somewhat more difficult to estimate the d-orbital contributions to the bonding, since the molecular ground states do not correlate to the d"dmsu2states which characterize the coinage metal diatomics, but correspond instead to d"dmsu2su*1states. Nevertheless, the d-orbital contributions to the bonding in TiCo are 0.47 eV greater than in VNi, and it is argued that this is primarily a result of the larger size of the 3d orbitals in Ti and Co than in V and Ni, respectively.
V. Conclusions Abrupt predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiV, V2,TiCo, and VNi, permitting the bond strengths of these molecules to be determined as D,(TiV) = 2.068 f 0.001, D0(V2) = 2.753 f 0.001, Do(TiCo) = 2.401 f 0.001, and D,(VNi) = 2.100 f 0.001 eV. These moleculesjoin NiPt,18 and Pt2I4as transition-metal diatomics for which an abrupt predissociation threshold in an extremely congested electronic spectrum has been used to measure bond
Acknowledgment. We thank Jeff Bright for his expert preparation of the TiV, TiCo, and VNi alloys, and Professor William H. Breckenridge for the use of the intracavity etalon employed in high-resolution studies, which allowed the predissociation thresholds to be accurately measured using the absorption spectrum of I,. We gratefully acknowledge research support from the National Science Foundation under Grant CHE-8912673. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Zero-Fieid Splitting of the First Excited Triplet State of Dlbenzocycioheptadienyiidene. A Carbene to Biradical Transformation upon Electronic Excitation A. Despres, V. Lejeune, E. Migirdicyan,* Laboratoire de Photophysique MolPculaire du CNRS, B6timent 21 3, UniversitP de Paris-Sud, 91405 Orsay Cedex, France
and M. S. Platz Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received: October 15, 1991) The quasi-line fluorescence and excitation spectra of dibenzocycloheptadienylidene(DBC) in n-hexane at 20 K, obtained by site-selective laser experiments, do not present mirror-image symmetry. The fluorescence decay of matrix-isolated DBC is nonexponential and attributed to the emission from different sublevels of the first excited triplet state. In the presence of a magnetic field, the lifetime of the slow component decreases. Its dependence as a function of a weak magnetic field can be calculated for different values of the zero-field splitting parameter D. The best fitting value is ID1 = 0.02 cm-I. The D value in the first excited triplet state is significantly smaller than in the ground state where ID/is known to be 0.3932 cm-I. The decrease of the D value is interpreted with very simple molecular orbital theory. The excitation of DBC takes electron density off the carbene center and delocalizes it into the aromatic rings.
Introduction Conventional electron paramagnetic resonance (EPR) and the optically detected magnetic resonance (ODMR) methods have been commonly used to determine the zero-field splitting (ZFS) parameters D and E of triplet states having lifetimes longer than milliseconds. These techniques are, however, more difficult to apply to excited species having shorter triplet lifetimes since their
steady-state concentration is very low. In a recent paper,l we Presented a method that allows one to determine the ZFS Parameter D Of excited triplet states Of m-xylylene biradicals having lifetimes Of the order Of microseconds or shorter. The method (1) Lejeune, V.; Despres, A.; Migirdicyan, E. J . Phys. Chem. 1990, 94, 8861
0022-3654/92/2096-2486%03.00/00 1992 American Chemical Society
Excited Triplet State of Dibenzocycloheptadienylidene uses the magnetic field effect on the emission decays from triplet sublevels. In the case of 5-methyl-m-xylylene in the first excited triplet state T I , this method indicates that D = 0.04 cm-', a value which is significantly larger than the D = 0.011 cm-' value measured by EPR for the same biradical in the triplet ground state To.* The D parameter gives information about the distribution of unpaired electrons, since it is proportional to ( f 3 ) , r being the distance between the two unpaired electrons. The observed increase of the D parameter between To and T I , confirmed by SCF-CI ~alculations,~ suggests that the average distance ( r ) is significantly smaller in Ti. This is explained by a drift of the unpaired electrons from the extracyclic methylenes toward the benzene ring when the biradical is electronically excited. The aim of the present work is to extend the scheme established for m-xylylene biradicals to another class of ground-state triplet species, the aromatic carbenes, which also exhibit a triplet-triplet fluorescence. Recent years have witnessed dramatic progress in the spectroscopy and dynamics of aromatic carbenes in both their ground triplet and in electronically excited triplet states. Laser flash photolysis studies have determined the absolute rate constants for reaction of these species with hydrogen and chlorine atom donors.4 It has recently been possible to isolate some aromatic carbenes such as dibenzocycloheptadienylidene (DBC) in Shpolskii type matrices at cryogenic temperatures and thereby obtain highly resolved fluorescence from the excited triplet state.5 The non-
a DBC
exponential fluorescence decay and its change under a static magnetic field have been reported as In the triplet ground state To, the ZFS parameter D = 0.3932 cm-I of DBC has been determined by EPRe6 Here we report the D value of this carbene in the first excited triplet state T,. This parameter has been estimated from the magnetic field effect on the emission decays from triplet sublevels.
Experimental Section Materials. The diazo precursor of DBC was prepared as previously described.' n-Hexane (Merck Uvasol) was used without further purification. Experimental Setup. DBC was generated in situ by photolysis of its diazo precursor dispersed in n-hexane at 15-20 K with 315-nm radiation (isolated from a 150-W Osram high-pressure xenon lamp through a silica prism Jobin-Yvon monochromator). The spectra were obtained with quickly frozen samples. Rapid cooling is expected to isolate the diazo precursor and consequently DBC in substitutional sites of the matrix. The samples were mounted on a holder inside an Air Liquide cryostat which used cold helium as coolant. The excitation source was a homebuilt tunable dye laser pumped with a Lambda-Physik M 2000 nitrogen laser. A solution of coumarin 480 in ethanol was used as a lasing medium and the laser line width was 6 cm-'. The site-selected fluorescence spectra of DBC were analyzed with a T H R 1500 Jobin-Yvon (1.5 m, f/12) scanning spectrometer; spurious light was eliminated by using gated detection (2) Wright, B.; Platz, M. S. J. Am. Chem. SOC.1983, 105, 628. (3) Lejeune, V.; Berthier, G.; Desprts, A.; Migirdicyan, E. J. Phys. Chem. 1991. 95. 3895. (4) For a review of the ground-state behavior of triplet arylcarbenes see: Platz, M.S.; Maloney, V. M. Kinetics and Spectroscopy of Carbenes and Eiradicals; Platz, M. S., Ed.; Plenum: New York, 1990; p 239. For a review of their triplet excited states see: Scaiano, J. C. Ibid. p 353. ( 5 ) Haider, K. W.; Platz, M. S.; Despres, A.; Lejeune, V.; Migirdicyan, E. J. Phys. Chem. 1990, 94, 142. (6) Moritani, I.; Murahashi, S.I.; Nishino, M.; Yamamato, Y.; Itoh, K.; Mataga, N. J. Am. Chem. Soc. 1967, 89, 1259. (7) (a) Moritani, I.; Murahashi, S. I.; Nishino, M.; Kimura, K.; Tsubomura, H.Tetrahedron Lett. 1966, 373. (b) Moritani, I.; Murahashi, S.I.; Ashitaka, H.; Kimura, K.; Tsubomura, H. J. Am. Chem. Soc. 1981,103,6373.
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2487 r
-
-
.- -
-
-.
___
0,o
502.9 run
EXCITATION
I
475
500
FLUORESCENCE
1 525
550
Figure 1. Site-selected fluorescence and excitation spectra of DBC in n-hexane at 20 K. The emissions I and I1 are respectively excited with 486.5- and 488-nm laser lines. The excitation spectra I11 and IV are obtained by monitoring the fluorescence origins at 502.9 and 504.7 nm, respectively, while scanning the dye laser.
TABLE I: Observed Vibrational Frequencies in the To and TI States of DBC in n-Hexane at 20 K vibrational frequency in cm-I in state To in state T , 625 695 670 940 1000 1040 1170 1165 1230 1580 1530
tentative assignment 6a 12 (CCC bending) 1
C-CHZ 8a
(Stanford Research SRS 250 boxcar with time gate 100-400 ns). The site-selected fluorescence excitation spectra were obtained by monitoring the intense fluorescence origin bands through the THR spectrometer while scanning the dye laser. The spectra were not corrected for variation in laser intensity. The fluorescence decays were excited with the 337 nm radiation of the nitrogen laser and measured by using equipment and techniques as previously described.*
Results A. Fluorescence and Excitation Spectra. In a previous report? we presented the fluorescence of DBC in n-hexane at 12 K excited with 315-nm radiation (see Figure 5, ref 5). The spectrum consists of sharp zero-phonon lines followed on their long-wavelength side by broad, structured phonon sidebands. The fluorescence starting at 502.9 nm contains a sharp line 71 cm-' removed from the origin. This sharp line at 504.7 nm could be attributed either to a lowfrequency vibronic band corresponding to a hindered motion of the seven-membered ring or to the fluorescence origin of DBC in a different site. The following experiments carried out under selective narrow-band laser excitation provide evidence in favor of the second interpretation. The fluorescence spectrum of DBC in n-hexane at 20 K excited with a laser line a t 486.5 nm is displayed in Figure 1, curve I. This spectrum (I) starting at 502.9 nm contains no sharp band at 504.7 nm. Alternatively the excitation of the sample with a laser line at 488 nm produces a fluorescence (11) starting at 504.7 nm and is presented in Figure 1, curve 11. The vibronic structures of spectra I and I1 are similar and the corresponding ground state To vibrational frequencies are given in Table I. A splitting of (8) Lejeune, V.; Desprts, A.; Fourmann, B.; Benoist d'Azy, 0.;Migirdicyan, E. J. Phys. Chem. 1987, 91, 6620.
2488 The Journal of Physical Chemistry, Vol. 96, No. 6 , 1992
Despres et al.
1
0 400 800 t(ns 1 Figure 2. Fluorescence decay of DBC in n-hexane at 20 K excited with the 337-nm nitrogen laser line and measured on the intense fluorescence origin band at 502.9 nm.
30 cm-l is observed on bands at 940,1040, and 1170 cm-I removed from the origins. DBC, like 1,4-cycloheptadiene? can exist in two different nonplanar conformations C2and C,. It is reasonable to assume Y
G c, that the vibrational structures of the spectra corresponding to conformers C2 and C, would be different. Since we observe the same structures in spectra I and 11, we attribute them to a given conformer of DBC in two different sites of the n-hexane matrix. Conformer C2 is expected to be more stable than conformer C, for steric reasonslO and by *-orbital conjugation. As the angle between the two benzene rings is smaller in C2than in C,, the diazo precursor is more likely trapped in conformation C, during preparation of the frozen sample. The laser-induced fluorescence excitation spectra 111 and IV obtained by monitoring the two fluorescence origins at 502.9 and 504.7 nm, respectively, are displayed in Figure 1, curves 111 and IV. The spectra are composed of sharp zero-phonon lines accompanied on the short-wavelength side by broad structured phonon side bands. Since the fluorescence spectrum I contributes in part to the intensity of the fluorescence origin of spectrum I1 at 504.7 nm, the excitation spectrum IV contains some bands (labeled with an asterisk (*)) belonging to spectrum 111. Except for these labeled bands, the vibronic structures of spectra 111 and IV are similar and the vibrational frequencies of the first excited triplet TI are listed in Table I. This table also contains a tentative assignment of DBC vibrations based on the vibrational frequencies of benzene. Except for mode 8a which appears at 1580 cm-I in fluorescence and at 1530 cm-' in the excitation spectrum, particularly noteworthy is the absence of mirror image symmetry between the fluorescence and excitation spectra of DBC in a given site. For example, in the 600-700-cm-' region, there are two bands of equal intensity in fluorescence and only one band in the excitation spectrum. Similarly, around 1000 cm-', there are two fluorescence bands and only one excitation band. Around 1200 cm-I, on the other hand, there are two excitation bands and only one fluorescence band. This spectral asymmetry suggests the existence of vibronic coupling between the emitting triplet and another excited triplet state of DBC. In order to understand the mechanism of this vibronic coupling, it would be useful to calculate the transition energies and oscillator strengths of the whole set of excited triplet states of DBC. (9) Saebo, S.; Boggs, J. E. Theochem 1982, 4 , 365. (10) Burkett, U.;Allinger, N. L. Molecular Mechanics, ACS Monograph 177; American Chemical Society: Washington, DC, 1982.
0 400 800 t(ns ) Figure 3. Fluorescence decays of DBC in n-hexane at 20 K in the absence (a) and in the presence of 45,65, 105, and 145 G (b-e) magnetic fields. The decays are excited with the 337-nm nitrogen laser line and measured on the intense fluorescence origin band at 502.9 nm. B. Fluorescence Decays. 1. In the Absence of Magnetic Field. The fluorescence decay of DBC in n-hexane at 20 K excited with 337-nm nitrogen laser radiation and measured on the origin band at 502.9 nm is presented in Figure 2. On this semilogarithmic scale, it appears that the decay is nonexponential. It has been analyzed as a sum of three exponential decays with the same preexponential factor AI1
The zero-field splittings of DBC in the ground state6 are much smaller than kT so that the three triplet sublevels are equally populated a t 20 K. This equal population is maintained during the absorption process which conserves spin orientation, since the 6-11s duration of our excitation pulse is short compared to the component lifetimes of the decay. This is why the decay curve of DBC can be analyzed by using the same preexponential factor A for the three components. This treatment leads to component lifetimes T~ of 35,67, and 140 ns, in reasonably good agreement with our previous results5 obtained for a slowly cooled sample in n-heptane at 20 K. The lifetime of the slow component is determined within an uncertainty of 7%. 2. In the Presence of a Magnetic Field. Upon application of a static magnetic field, the fluorescence decay of DBC in n-hexane at 20 K is significantly altered, as was the case for DBC in an n-heptane matrix? The fluorescence decays corresponding to fields of increasing intensity H = 25,65, 105, and 145 G are presented in Figure 3. As for m-xylylene biradicals? the nonexponential fluorescence decay of DBC is attributed to the emission from the TI sublevels, at a rate faster than the rate of spin-lattice relaxation between the different sublevels. The modification of the decay curves upon application of the field is due to the mixing of the wave functions of the TI sublevels. The lifetime of each mixed sublevel depends upon the orientation of molecular axes with respect to the field direction. For randomly oriented DBC in a polycrystalline Shpolskii matrix, the decay law is expected to be multiexponential. In the presence of a weak magnetic field, however, the decay curves are still fitted with a sum of three exponential functions, each lifetime assumed to be the central value of a narrow distribution. The results of such an analysis, displayed in Figure 4, indicate that the lifetime T" of the slow component decreases significantly as the field H increases up to about 200 G. The variations with H of the two faster components lifetimes are within error bars. This magnetic field effect on T H is very similar to that observed for m-xylylene biradicalsl and will therefore be used to estimate the ZFS parameter D of DBC. 3. Interpretation. The fluorescence of DBC corresponds to the spin-allowed transition between TI and To states. Since the transition moments are spin-independent, the radiative decay rate (11) Lejeune, V.;Desprts, A.; Migirdicyan, E.; Siebrand, W. J . Phys.
Chem. 1991, 95, 7585.
Excited Triplet State of Dibenzocycloheptadienylidene
The Journal of Physical Chemistry, Vol. 96,No. 6,1992 2489 is significantly larger in the TI state than in the To state. This is explained by a delocalization of the A electron on the aromatic rings.
l2OI
\+ \
)i
I
100
0
200
H (Gauss 1 Figure 4. Lifetimes of the slow component in the fluorescence decay of DBC in n-hexane at 20 K as a function of a weak magnetic field. The crosses correspond to the observed values. The curves have been calculated for two values of the ZFS parameter IDJ:(a) ID1 = 0.020 cm-I; (b) ID1 = 0.015 cm-l.
constants kr from the three sublevels are equal in a purely Coulombic approach, In contrast, magnetic interactions distinguish between the three sublevels having different symmetries, each sublevel being contaminated by a different singlet state through spin-orbit coupling (SOC). For example, in m-xylylene biradicals having C,, symmetry with y and z axes in the molecular plane, only the Y and Z sublevels (which are antisymmetric with respect to the molecular plane) can interact with the higher-lying (.A)* states. This SOC induces intersystem crossing (ISC) which accelerates the depopulation of Y and 2 sublevels of the TI state to the lower singlet states. The X sublevel, which cannot interact with ( U T ) * singlets, will give rise to the slow component with a total decay rate kr k? < k' + k&,
+
As per the m-xylylene biradicals, the nonexponential fluorescence decay of DBC will be interpreted by different ISC rates from the three triplet sublevels, due to different SOC with the singlet states. This interpretation is confirmed by the significant modification of the fluorescence decays observed in the presence of a magnetic field. 4. Determination of the ZFS Parameter Din the TIState. The effect of the Zeeman mixing on the radiative and on the nonradiative decay rates of the TI-To transition has already been discussed in detail for m-xylylene biradicals.] This has led to the following expression for the effective total decay rate (k x H )from the X sublevel (slow component) in the presence of a weak field
where k,, k,,, and k, are the total decay rates from the X,Y, and Z sublevels in the absence of the field. In the case of DBC excited in the T, state, we will provisionally assume that the slow component (rate k,) can be associated with the X sublevel separated by D from the two others, as in the case of m-xylylene. DBC is nonplanar, so the correspondingX magnetic axis should be associated with the normal to some average plane in a quasi-planar conformation (as in conformer C2), We can then estimate the ZFS parameter D of DBC in the TI state from expression 1 where every quantity except D is known from our decay measurements. The dependence of TEI = (kxH)-las a function of a weak magnetic field has been calculated for different D values by using expression 1. The corresponding curves for 7;' with two tentative values of ID1 are displayed in Figure 4. The best fit between the calculated curve and experimental points corresponds to ]Dl = 0.020 f 0.005 cm-'. The D value of the TI state is significantly smaller than the value ID1 = 0.3932 cm-l measured by EPR6 for DBC in the ground state To. The decrease of the D value indicates that the average separation between the two unpaired electrons
Discussion Localization versus Delocalization. The aromatic carbenes in the ground To state are characterized by a large D value due to the localization of the two unpaired electrons on the two atomic orbitals (AO) of the central divalent carbon;these A 0 are directed respectively along t h e y axis of C2 symmetry and the normal x to the C-C-C plane. This localization also explains why the axis associated with the largest zero-field splitting is foundI2 oriented along z for diphenylcarbene (DPC) where ID1 = 0.4050 cm-'. Since a similar large ID1 = 0.3932 cm-' value is also observed for DBC, the magnetic axes are presumably the same. For both species in the ground To state, the D parameter measures the energy separation between the Z sublevel and the two others. If the same localization of unpaired electrons were preserved in the excited TI state, large D separations would require field intensities of several thousand gauss, instead of a few hundred, in order to observe sizable magnetic effects on the fluorescence decays. In addition, SOC predictions would make the Z sublevel short-lived and the two others long-lived, which is contrary to our observations where the slow component is well-separated. The small ID1 = 0.02 cm-I value obtained in the present study for DBC is explained by the delocalization of the ?r electron on the aromatic rings in the TI state. The excited carbene thus behaves like aromatic molecules13 and m-xylylene biradicalsl I where the normal to the molecular plane is associated with the smallest SOC matrix element. For DBC in the TI state, the smallest nonradiative rate will also be associated with x , the normal to the average plane in the quasi-planar conformation C2 (the magnetic axes x and z are assumed to be the same in the To and T, states). In such molecules, the D parameter measures the energy separation in the TI state between the X sublevel and the two others, instead of the Z sublevel in the ground Tostate. Consequently, expression 1, where (k,H) is the total decay rate of the slow component from the X sublevel, can indeed be used to estimate the ZFS parameter D. Hiickel Approach. In an aromatic carbene, the major contributor to 1 0 1 is the interaction of the unpaired electron in the localized u orbital of the carbene with the A electron density in the p, orbital centered on the carbene carbon. X
Our results imply that the r-electron density at the carbene carbon is much smaller in the first excited TI state than in the ground To state. This interpretation is consistent with very simple molecular orbital theory. A portion of the 13 Hiickel ?r MOs of the diphenylmethyl radical, our model of the A system of DBC, are given below. Energy YlO - - T8
I 9
tt** y4
y5
y0
The coefficient of the singly occupied MO (energy = a) at the diphenylmethyl central carbon is 0.632. An excited state of (12) Brandon, R. W.; Closs, G . L.; Hutchison Jr., C. A. J . Chem. Phys.
1962, 37, 1878. (13) Schmidt, J.; Antheunis, D.; van de Waals, J. H. Mol. Phys. 1970.22, 1.
J. Phys. Chem. 1992, 96, 2490-2494
2490
diphenylmethyl radical is obtained either by promotion of an electron from the doubly occupied q 4 . 5 . 6 orbital to \k7, or by promotion of one electron in \k7 to P8,9,10. This will produce a singly occupied MO in either 9 4 , 5 , 6 or \k8,9,10.The coefficients of these MO's at the diphenylmethyl radical center are all equal to zero!I4 Thus excitation of diphenylmethyl (or DBC) takes electron spin density off the diphenylmethyl radical (or carbene center) and places it into aromatic rings. This has the effect of dramatically reducing ID1 in DBC in the excited TI state.
Conclusion Theory and experiment agree that, in aromatic carbenes, the (14) Dicrionury of T Electron Culculurions, Coulson, C . A,, Streitwieser, Jr., A., Eds.; W. H. Freeman: New York, 1965.
-
distribution of unpaired electrons changes considerably upon To TI excitation. In the first excited triplet state T I , the spin distribution of the aromatic carbene becomes similar to that of a biradical with one unpaired electron on the aromatic ring and the other on the extracyclic carbene carbon. This distribution of unpaired electrons is very similar to that predicted by D value calculations of the m-xylylene biradical in the first excited triplet state TIS3Thus, it is not surprising that the D value for DBC in T I (D= 0.02 cm-l) is close to the value D = 0.04 cm-' found for m-xylylene biradical, in the first excited triplet state TI, I Acknowledgment. We thank Professor Joseph Michl for valuable discussions. Registry No. DBC, 15306-40-8.
( 1,2)-Hydrogen Shift In Monovalent Carbon Compounds: The Methylcarbyne-Vinyl Radical Isomerization Ida M. B. Nielsen, Curtis L. Janssen, Neil A. Burton, and Henry F. Schaefer III* Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602 (Received: October 28, 1991)
-
Ab initio molecular electronic structure theory has been applied in an investigation of the methylcarbyne-vinyl radical isomerization, (CH3C CH,CH). The reaction has been studied on both the ground-state (doublet) and lowest-lying quartet-state potential energy hypersurfaces. Methylcarbyne, the vinyl radical, and the transition state connecting them have been located using the self-consistentfield (SCF) and configuration interaction including all single and double excitations (CISD) methods with several basis sets, the largest being of triple f plus double polarization (TZ+ZP) quality. Harmonic vibrational frequencies have been obtained at the SCF level of theory, and the stationary points characterized as minima or transition states. The coupled cluster method with single and double excitations (CCSD) and with connected triple excitations as well (CCSD(T)) has been used for computing single-point energies with the TZ+ZP basis set, and single-point energies have also been computed at the CISD+Q level using a large atomic natural orbital basis set. Classical barriers to isomerization of 9 and 57 kcal/mol have been obtained for the doublet and quartet state, respectively, at the highest level of theory. The true gound state activation energy is estimated to be 6 kcal/mol, or about 4 kcal/mol higher than that for the well-characterized vinylidene species.
Introduction Methylcarbyne (CH3C) is among the simplest of monovalent carbon compounds R-C. Methylcarbyne is suggested to be fundamentally responsible for the preparation of the primary chemiion, CH3CO+,which is formed when reacting O(3P) with 2-butyne and, to a lesser extent, with 1-butyne.' Following the production of methylcarbyne from 1-butyne or 2-butyne, the proposed elementary reaction is CH3C + O(3P) CH3CO+ + e(1) However, the isomerization of methylcarbyne to the vinyl radical (CH*CH) CH3C CH2CH (2) might be fast enough to prevent methylcarbyne from participating in reaction 1. Studies of the analogous monovalent carbon reaction CH + O(3P) CHO+ + e(3) present evidence that the reacting species C H is not in the ground state but in the metastable a4Z state.2 In a like manner, methylcarbyne in reaction 1 could be in a quartet state, in which case there would be two possible pathways for the methylcarbyne rearrangement: either rearrangement of the quartet-state methylcarbyne to a quartet state of the vinyl radical followed by an intersystem crossing to the ground-state (doublet) vinyl radical,
-
-
Corresponding author.
or an intersystem crossing from quartet to doublet methylcarbyne followed by a rearrangement to the vinyl radical on the doublet potentiabenergy surface. The goal of the present study is to provide insight into the methylcarbyne-vinyl radical isomerization on the doublet as well as on the quartet potential energy surface. The ground-state (doublet) and lowest-lying quartet state of methylcarbyne, the vinyl radical, and the transition states that connect them are located, and the barriers to the arrangements are predicted. The methylcarbyne-vinyl radical isomerization 2 has, to our knowledge, never been studied previously, despite its obvious fundamental importance to combustion chemistry. However, other reactions of this general type, Le., (1,2)-hydrogen migrations in hydrocarbons, have been investigated, including (1,2)-hydrogen migration to the radical center in the vinyl radical, the ethyl radical, and singlet and triplet methyl~arbene.~.~ Also, the hydrogen migration in singlet and triplet vinylidene has been studied e x t e n s i ~ e l y . ~For ~ ~the vinylidene and methylcarbene singlet and (1) Vinckier, C.; Gardner, M. P.; Bayes, K. D. Sixteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1977;
p 881. (2) Bayes, K. D. Chem. Phys. Lett. 1988, 152, 424. (3) Harding, L. B. J . Am. Chem. SOC.1981, 103, 7469. (4) See, for example: Altmann, J. A,; Csizmadia, I. G.;Yates, K. J . Am. Chem. Soc. 1974,96,4196. No&, R. H.; Radom, L.; Rcdwell, W. R. Chem. Phys. Left.1980, 74, 269.
OO22-3654/92/2096-2490%03.00/O 0 1992 American Chemical Society
