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Moderate-size basis set ab initio calculations for the pi-electron vertical ionization potentials of trans- and cyclobutadiene (116 and 128 functions, respectively) are performed using the effective valence shell Hamiltonian (H(nu)) method. These calculations demonstrate the flexibility and unique features of the H(nu) method because the ionization potentials emerge from previous computations of the neutral valence-like and Rydberg state excitation energies using the same H(nu). The computed ionization potentials agree well with experiment and with previous calculations, including highly correlated ones for trans-butadiene. New predictions are provided for ionizations to quartet and shake-up states. Furthermore, in contrast to current beliefs about quasidegenerate perturbation theory, the third-order H(nu) calculations do not encounter intruder state problems.

Semiempirical molecular dynamics is used to study the collision of C60 and C+60. Particles are propagated classically using forces calculated from the modified neglect of differential overlap (MNDO) Hamiltonian. By assigning different collision energies (Ec) and impact parameters (b) to the Buckminster fullerenes (buckyballs), we simulated six collision events: four head on collisions with impact parameter b=0 and collisions energies of 100, 150, 200, and 400 eV, and two collisions with b=1.5 Rb (Rb=buckyball radius) and Ec=100 and 400 eV. The head on collisions show that at 100 eV the two buckyballs scatter off one another and at 200 and 400 eV they fuse, while at 150 eV they either scatter or form a metastable dimer depending on how the simulation is prepared. This barrier is consistent with recent experiments. In addition, we observe tetrahedral bonding in the C+120 structure formed in the 200 eV, b=0 collision, while at 400 eV, b=0 we see large rings and chains of carbon atoms. The off center collisions also display interesting structural features. In the 100 eV b=1.5 Rb collision, the buckyballs graze one another, distorting their cage structure and scattering at an angle relative to their incident velocities. The buckyballs in the 400 eV, b=1.5 Rb collision also scatter, but in contrast to the 100 eV, b=1.5 Rb collision, the individual buckyballs are severely distorted, forming what we refer to as an ‘‘open mouth’’ structure.

We consider the statistical properties associated with the packing of p self-avoiding rods of length M on a d-dimensional hypercubic lattice with N sites and periodic boundary conditions. The exact treatment for few (p≤4) rods is combined with information derivable from the lattice cluster theory (LCT) to obtain the exact analytic form for the free energy f per site. The thermodynamic limit of this free energy f is reexpressed as a series expansion about the zeroth order Flory mean field approximation. The expansion is in powers of the rod volume fraction φ=pM/N and contributions are retained through order φp (with p=4) for any M and d. The theory is compared with previous diagram based LCT calculations and with the DiMarzio approximation. Departures (in the thermodynamic limit) from the latter successful approximation appear at order φ4 and arise from correlations of four rods in configurations where the rods are not all parallel, correlations which are absent in the DiMarzio approximation. Our method uses computer enumerations to replace the time consuming task of evaluating the many-body diagrams of the LCT. The series for d≳1 are ill behaved in the large M limit and strongly indicate that resummations are required to obtain physically meaningful results.

The ground and first and second 1A″ potential surfaces of methyl mercaptan (CH3SH) are calculated as a function of the C–S and S–H bond coordinates using the ab initio effective valence shell Hamiltonian (Hν) method. The computations for this highly nontrivial system provide the first serious tests for choosing restricted valence spaces and for computing global potential energy surfaces with the Hν methods. The quasidegeneracy constraints on the Hν method suggest choosing a valence space which consists of the two (3a″ and 10a′) highest energy occupied orbitals in the ground state and the three lowest a′ unoccupied orbitals. The global potential surfaces are computed with a modest basis, but larger basis set computations at selected geometries test convergence for vertical excitation energies, ionization potentials, and C–S and S–H bond energies. The calculations are compared to both experiment and other calculations for this system. The computations assist in the interpretation of CH3SH photodissociation dynamics observed by Butler and co-workers.

The effective valence shell Hamiltonian (H) method is used to examine the electronic structure of cyclobutadiene. These computations are designed to help guide future experimental studies on this elusive compound, as well as aid in understanding the general features of effective Hamiltonian calculations. Calculations are performed with two qualitatively different valence spaces. The first valence space mimics the valence spaces used in semiempirical methods in that only four pi valence-like molecular orbitals are used. The second valence space includes four additional diffuse pi molecular orbitals. A [4s5p1d/2s1p] Cartesian Gaussian basis set is used (116 total functions). Our results agree with available spectroscopic vertical ionization potentials to within 0.1 and 0.3 eV for the b2g and b1u orbital ionizations. The calculated vertical excitation energies and ground state automerization barrier height agree well with previous ab initio calculations. For the first time, however, the lowest lying optical transition from the X 1A(g) ground state is predicted to occur at 5.99 eV to a Rydberg B-1(3u) state.

These experiments use molecular photodissociation of CH3SH to probe the dynamics and the influence of nonadiabatic coupling in the transition state region of the CH3+SH→CH3S+H reaction. Photoexcitation at 222 and 248 nm in the first of two absorption bands accesses the lower of the two coupled potential energy surfaces near the saddle point of the excited state reaction coordinate. Measurement of the resulting photofragments’ velocities and angular distributions determine the branching between the CH3+SH and the CH3S+H exit channels. At all wavelengths within the first absorption band, we observe preferential fission of the stronger S–H bond over the weaker C–S bond. Fission of the C–S bond occurs only to a small degree at 222 nm and is not observable at 248 nm. Comparison with our earlier data at 193 nm, corresponding to excitation to the upper bound adiabat which is nonadiabatically coupled to the lower dissociative surface reached at 222 nm, shows that the branching ratio between C–S bond fission and S–H bond fission is a factor of eight larger at 193 nm. To probe the forces in the Franck–Condon region, we also measure the photoemission spectrum from dissociating CH3SH excited at 222 nm and compare it to the previous measurement at 193 nm. The 222 nm spectrum evidences emission into the S–H stretch and methyl stretch vibrations but not into C–S stretching modes, consistent with the dominance of S–H fission on the lower adiabat, while the 193 nm emission spectrum, reassigned here, has only a progression in the C–S stretch. The comparison of the spectra suggests a model in which stretching along the C–S coordinate on the bound upper state occurs as the amplitude couples nonadiabatically to the lower dissociative surface, allowing the molecule to access the region near the saddle point on the lower surface at extended C–S bond lengths. This results in better overlap with the C–S fission exit channel and thus an increased branching to C–S bond fission over that observed upon direct excitation to the lower dissociative surface at 222 nm. To further advance the experimental conclusions, we present collaborative calculations of the potential energy surfaces using the effective valence-shell Hamiltonian method developed by Freed and co-workers.

The effective valence shell Hamiltonian (Hν) method is used to examine the electronic structure of cyclobutadiene. These computations are designed to help guide future experimental studies on this elusive compound, as well as aid in understanding the general features of effective Hamiltonian calculations. Calculations are performed with two qualitatively different valence spaces. The first valence space mimics the valence spaces used in semiempirical methods in that only four π valence-like molecular orbitals are used. The second valence space includes four additional diffuse π molecular orbitals. A [4s5p1d/2s1p] Cartesian Gaussian basis set is used (116 total functions). Our results agree with available spectroscopic vertical ionization potentials to within 0.1 and 0.3 eV for the b2g and b1u orbital ionizations. The calculated vertical excitation energies and ground state automerization barrier height agree well with previous ab initio calculations. For the first time, however, the lowest lying optical transition from the X 1Ag ground state is predicted to occur at 5.99 eV to a Rydberg 1B3u state.