We present calculations for the nonbonded interactions in the dimeric complexes:  methane dimer, ammonia dimer, water dimer, H2O·(NH3), CH4·(NH3), and (FHF)- as a function of theory level (HF, DFT(B3LYP), MP2, LMP2, MP3, MP4, CCSD(T), and others) and basis set (6-31G**, cc-pVXZ, X = D, T, Q, 5). Dimer minimum energy structures are determined at the MP2 theory level for the cc-pVTZ basis set employing analytical second derivatives. For HF and DFT levels of theory, methane dimer and one structure of CH4·(NH3) are not bound. The basis set superposition error (BSSE) begins to converge (becomes systematically small) for basis sets larger than cc-pVTZ. For hydrogen-bonded systems, most levels of theory seem to give reasonable estimates of the experimentally known binding energies, but here, too, the BSSE overwhelms the reliability of the binding energies for the smaller basis sets. The CH4·(NH3) dimer has two minimum energy conformations with similar binding energies, but very different BSSE values especially for small basis sets (cc-pVXZ, X ≤ T). On the basis of these calculations, we present a discussion of ab initio calculations of nonbonded interactions for molecules, such as phenethylamine, that have different conformations. Suggestions for possible next steps in the calculation of nonbonded interactions are presented.

The two tautomeric forms of 2-hydroxypyridine (2-HP) have been studied in a supersonic jet expansion. Time-of-flight mass spectroscopy (TOFMS) and emission spectroscopy of the lactim and lactam tautomers have been studied and are reported here. The lactim spectrum is similar to an earlier TOFMS spectrum and has its origin at 36 136 cm'1 11. Evidence of mixing of the * and * electronic states is seen in the lactim spectrum. The mixing is removed in the disolvate water cluster but not in monosolvate clusters of ammonia or water. The lactam is shown to be nonplanar giving rise to two origins in the excitation spectrum at 29 832 and 29 935 cm'1. The ammonia and water lactim cluster spectra show significant shifts to the red while the cluster spectra of the lactam show large shifts to the blue. Experimental evidence for strong hydrogen bonding in these clusters is discussed. Intramolecular and intermolecular proton transfer in 2-HP and its clusters is discussed in the context of these data.

Reactions of small neutral iron oxide clusters (FeO(1-3) and Fe(2)O(4,5)) with carbon monoxide (CO) are investigated by experiments and first-principle calculations. The iron oxide clusters are generated by reaction of laser-ablation-generated iron plasma with O(2) in a supersonic expansion and are reacted with carbon monoxide in a fast flow reactor. Detection of the neutral clusters is through ionization with vacuum UV laser (118 nm) radiation and time-of-flight mass spectrometry. The FeO(2) and FeO(3) neutral clusters are reactive toward CO, whereas Fe(2)O(4), Fe(2)O(5), and possibly FeO are not reactive. A higher reactivity for FeO(2) [sigma(FeO(2) + CO) > 3 x 10(-17) cm(2)] than for FeO(3) [sigma(FeO(3) + CO) approximately 1 x 10(-17) cm(2)] is observed. Density functional theory (DFT) calculations are carried out to interpret the experimental observations and to generate the reaction mechanisms. The reaction pathways with negative or very small overall barriers are identified for CO oxidation by FeO(2) and FeO(3). The lower reactivity of FeO(3) with respect to FeO(2) may be related to a spin inversion process present in the reaction of FeO(3) with CO. Significant reaction barriers are calculated for the reactions of FeO and Fe(2)O(4-5) with CO. The DFT results are in good agreement with experimental observations. Molecular-level reaction mechanisms for CO oxidation by O(2), facilitated by condensed phase iron oxides as catalysts, are suggested.

Reactions of neutral vanadium oxide clusters with small hydrocarbons, namely C2H6, C2H4, and C2H2, are investigated by experiment and density functional theory (DFT) calculations. Single photon ionization through extreme ultraviolet (EUV, 46.9 nm, 26.5 eV) and vacuum ultraviolet (VUV, 118 nm, 10.5 eV) lasers is used to detect neutral cluster distributions and reaction products. The most stable vanadium oxide clusters VO2, V2O5, V3O7, V4O10, etc. tend to associate with C2H4 generating products V(m)O(n)C2H4. Oxygen-rich clusters VO3(V2O5)(n=0,1,2...), (e.g., VO3, V3O8, and V5O13) react with C2H4 molecules to cause a cleavage of the C=C bond of C2H4 to produce (V2O5)(n)VO2CH2 clusters. For the reactions of vanadium oxide clusters (V(m)O(n)) with C2H2 molecules, V(m)O(n)C2H2 are assigned as the major products of the association reactions. Additionally, a dehydration reaction for VO3 + C2H2 to produce VO2C2 is also identified. C2H6 molecules are quite stable toward reaction with neutral vanadium oxide clusters. Density functional theory calculations are employed to investigate association reactions for V2O5 + C2H(x). The observed relative reactivity of C2 hydrocarbons toward neutral vanadium oxide clusters is well interpreted by using the DFT calculated binding energies. DFT calculations of the pathways for VO3+C2H4 and VO3+C2H2 reaction systems indicate that the reactions VO3+C2H4 --> VO2CH2 + H2CO and VO3+C2H2 --> VO2C2 + H2O are thermodynamically favorable and overall barrierless at room temperature, in good agreement with the experimental observations.

Dispersed emission and time of flight mass spectra are presented for jet-cooled toluene, and 0-, m-, and p-xylene. The spectra exhibit features, typically within loo cm-I of the SI-.:t.S o origins, which are assigned to transitions associated with the internal rotation of the ring methyl groups. A model is developed which treats this methyl motion as that of a one-dimensional rigid rotor. The spacings of the peaks in the spectra are used to solve for the rotational constant B of the methyl rotor, and for the size and shape ofthe n-fold barrier to rotation (i.e., V 3 , V 6 , etc.) within this model. For toluene and p-xylene, the barrier is found to be small in both the ground (So) (V 6-10 cm-I) and excited (SI) (V 6-25 cm-I) electronic states. For m-xylene, the ground state is again found to have a low barrier (V6-25 cm-I), but the excited state has a potential barrier of V3 = 81 cm-I, V6 =-30 cm-I. The barrier to rotation of the ring methyl groups is observed to be the highest for a-xylene. In this case the ground state is found to have a rather large barrier V3 = 425 cm-I, V6 = 18 cm-I which changes to V3 _ 166 cm-I, V;-25 cm-I, and V6-0 cm-I in the excited state. The V; term represents a potential cross term between the two methyl rotors. The use of a kinetic energy cross term with a weighting coefficient of 0.72 in the Hamiltonian is also required for an accurate description of the excited state of this isomer. Empirical force field (EFF) calculations are performed for toluene and the three xylenes using a molecular orbital-molecular mechanics (MOMM) algorithm. The EFF-MOMM calculations are in essential agreement with the spectroscopic results and the one-dimensional rigid rotor model. II. EXPERIMENTAL PROCEDURES The time-of-flight mass spectrometer (TOFMS) chamber has been described previously.s TOFMS experiments utilize an R. M. Jordan pulsed valve. Helium is used

The optical absorption spectra of the first excited singlet states of the benzene, toluene, and toluene–benzene dimers, created in a supersonic molecular jet, are reported. The absorption spectra are detected through two-color time of flight mass spectroscopy; this method eliminates fragmentation of dimers and higher clusters and the dimer spectra are uniquely observed. The benzene dimer observed in this experiment is suggested to have a parallel stacked and displaced configuration of C2h symmetry. Both the toluene and toluene–benzene dimers have two configurations: parallel stacked and displaced [based on (benzene)2] and perpendicular. (Benzene)2, (toluene)2, and toluene–benzene form excimers in the excited state for the parallel stacked displaced configurations. The transformation of (benzene)2 to the excimer takes place at the 00 with a ∼0 cm−1 barrier while the excimer is formed for toluene–benzene with a ∼900 cm−1 barrier. An exciton analysis of the (benzene)2 000 and 610 yields M12, the excitation exchange interaction, equal to ∼1.6 cm−1.

More than a dozen cases of nonrigid van der Waals clusters are presented and discussed to demonstrate that cluster nonrigidity is a general phenomenon in all weakly bound systems. The interplay of structure and nonrigidity complicates cluster research and mandates a dynamical approach to cluster properties in which multiple stable configurations coexist and interconvert, and large amplitude nuclear motions are the rule rather than the exception. Empirical potential energy surface calculations are employed to yield physical insight into the structure and dynamics of nonrigid clusters, and molecular symmetry group theory is applied to analyze spectroscopic manifestations of cluster nonrigidity. Empirical potentials of various forms are successful in predicting most cluster structures, as well as estimating the potential surface barrier heights hindering the interconversion between different local minimum-energy structures. Such calculational approaches also emphasize the importance of large amplitude motion for one or more of the cluster vibrational degrees of freedom. The limits of these empirical calculations are discussed, and recent attempts to derive cluster structure and properties by ab initio techniques are reviewed. The aromatic/small molecule clusters considered in this presentation display two types of nonrigidity:  local nonrigidity in which large amplitude motion involves the rotation of one of the molecular constituents, and global nonrigidity in which large amplitude motion involves displacement of the centers of mass of the molecular constituents. The former motion interchanges equivalent atoms, and the latter motion interchanges cluster conformations. The potential surface barriers for these large amplitude motions tend to increase with constituent molecule complexity.

In order to elucidate the difference between nitramine energetic materials, such as RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), and CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), and their nonenergetic model systems, including 1,4-dinitropiperazine, nitropiperidine, nitropyrrolidine, and dimethylnitramine, both nanosecond mass resolved excitation spectroscopy and femtosecond pump-probe spectroscopy in the UV spectral region have been employed to investigate the mechanisms and dynamics of the excited electronic state photodissociation of these materials. The NO molecule is an initial decomposition product of all systems. The NO molecule from the decomposition of energetic materials displays cold rotational and hot vibrational spectral structures. Conversely, the NO molecule from the decomposition of model systems shows relatively hot rotational and cold vibrational spectra. In addition, the intensity of the NO ion signal from energetic materials is proportional to the number of nitramine functional groups in the molecule. Based upon experimental observations and theoretical calculations of the potential energy surface for these systems, we suggest that energetic materials dissociate from ground electronic states after internal conversion from their first excited states, and model systems dissociate from their first excited states. In both cases a nitro-nitrite isomerization is suggested to be part of the decomposition mechanism. Parent ions of dimethylnitramine and nitropyrrolidine are observed in femtosecond experiments. All the other molecules generate NO as a decomposition product even in the femtosecond time regime. The dynamics of the formation of the NO product is faster than 180 fs, which is equivalent to the time duration of our laser pulse.