The triplet-triplet fluorescence of perprotonated and perdeuterated diphenylcarbene (DPC-hlo and -dlo) in n-hexane at 4.2-25 K are both composed of a sharp spectrum and a broad blue-shifted one. They are attributed to two different conformers, the sharp spectrum corresponding to DPC in a quasi-planar geometry. The excitation spectra of both emissions consist of broad bands with a large Stokes shift. A detailed CS-INDO CI investigation (configuration interaction based on an intermediate neglect differential overlap method adapted to conformation and spectroscopy problems) is performed for threedifferent geometriesof DPC having CZ,, C2, and Cl symmetries. The computed triplet-triplet transition energies which indicate an hypsochromic shift going from planar CZ, to nonplanar C1 structures compare favorably with the experimental data. The fluorescence decays of DPC-hlo and -dl0 are nonexponential and attributed to the emission from different sublevels of the first excited triplet state. The measured decays of the broad fluorescence are longer than those of the sharp one. This is in agreement with CS-INDO CI calculations which predict a decrease of the oscillator strength of the TFT1 transition going from CZ, to C1 structures, as due to the inversion between the two excited triplets corresponding to the n r * 1 and nr*2 configurations. The zero-field splitting (ZFS) parameter D(T1) of DPC in the first excited triplet T I has been estimated from the magnetic field effect on the fluorescence decays. D(T1) = 0.007 and 0.20 cm-l for measurements carried out respectively on the sharp and on the broad fluorescence. Tentative ZFS calculations give D(T1) = 0.0155 cm-l for the Cb structure and D(T1) = 0.01 15 cm-l for the CI structure.The measured and calculated D(T1) values are always smaller than the D(T0) = 0.405 cm-1 value of DPC in the ground triplet state, which indicates the delocalization of the r electron density onto the aromatic rings upon excitation. It is therefore reasonable to detect a specific chemistry of this carbene in the presence of an intense laser which creates a high concentration of DPC in the first excited triplet T1.

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The quasi-line fluorescence and excitation spectra of 2-naphthylphenylcarbene (2-NPC) in n-hexane and n-heptane Shpolskii matrices at 4.2 K are composed of several spectra with origins shifted by 30-300 cm-' from each other. They have been attributed to the triplet-triplet transition of the syn and anti conformers of 2-NPC, each one being trapped in several different sites of the matrix. This attribution has been confirmed by the EPR experiments performed at 77 K. Furthermore, the fluorescence and EPR spectra reveal the lightinduced transformation of the syn 2-NPC into the anti 2-NPC conformer in the n-heptane matrix. The fluorescence decays of 2-NPC in both matrices at 4.2 K are nonexponential and attributed to the emission from different sublevels of the first excited triplet state TI. The zero-field splitting parameter D(T1) = 0.01 1 cm-' in the TI state of 2-NPC has been estimated from the magnetic field effect on the fluorescence decays. This value is much smaller than the D(T0) = 0.41 cm-' value in the ground state TO of 2-NPC determined by EPR, which indicates the delocalization of the z electron density onto the aromatic rings upon excitation.

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 Pt2 in 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,. V. ConclusionsAbrupt 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 molecules joinNiPt,18 and Pt2I4 as transition-metal diatomics for which an abrupt predissociation threshold in an extremely congested electronic spectrum has been used to measure bond 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-orbitalcontributions t...

The zero-field splitting (zfs) parameter Z> of m-xylylene biradical in the ground triplet and the two first excited triplets is calculated by two SCF-CI methods where overlap of atomic orbitals and multicentric zfs integrals are first neglected and then included according to the Mulliken approximation. The data show a significant increase of the D value from the ground state to the lowest excited triplets in good agreement with our experimental results. This is described by a drift of the unpaired electrons from the extracyclic methylenes toward the benzene ring when m-xylylene is excited.