John E. Bushnell

Binding energies of Ti+(H2)1–6 clusters: Theory and experiment

Formation of Ti+(H2)n clusters (n=1–6) has been studied by both temperature-dependent equilibrium measurements and density functional theory (DFT). The successive binding energies (BDEs) were measured to be 7.5±0.5, 9.7±0.6, 9.3±0.7, 8.5±0.4, 8.2±0.4, and 8.7±0.4 kcal/mol for n=1–6, respectively. The relatively low value of the n=1 BDE is due to a curve crossing from the Ti+[a4F(sd2)] ground state to the Ti+[b4F(d3)] first excited asymptote with the addition of the first ligand. The first BDE is 10 kcal/mol when measured with respect to the excited state asymptote. This series of almost constant BDEs is unlike any other M+(H2)n series. The present DFT calculations show these relatively constant BDE values for the Ti+(H2)n clusters are due to an electronic occupation which allows the Ti+ ion to interact equally with up to six H2 ligands. Bond lengths, geometries, and vibrational frequencies from the DFT calculations are reported here for all clusters. The influence of basis set size and computational metho...

Conformational analysis of cyclo(Phe-Ser) by UV–UV and IR–UV double resonance spectroscopy and ab initio calculations

We present the resonant two-photon ionization (R2PI) spectra as well as the UV–UV and IR–UV double resonance spectra for the cyclic dipeptide Phe-Ser. The R2PI spectrum shows five strong transitions in the region of 37 500–37 900 cm−1. By performing UV–UV double resonance spectroscopy, we distinguished 5 different conformers. For each of these conformers, the origin is the most intense transition. In addition, we performed IR–UV double resonance measurements in the region 3200–3800 cm−1 to analyse the NH and OH modes of each conformer. We compared the measured IR spectra to frequencies from ab initio calculations to assign each conformational structure. We found two structures in which the hydroxyl group of the serine residue forms a strong hydrogen bond with the carboxyl group of the same residue. One structure shows only a weak hydrogen bond and for the remaining two structures, the hydroxyl group is ‘free’.

Activation of methane by Ti+:: A cluster assisted mechanism for σ-bond activation, experiment, and theory1

Abstract Reactions of Ti+ with methane were studied by both temperature-dependent equilibrium measurements and density functional theory. Experimentally, we observed Ti(CH4)n+ clusters (n = 1–5) and the H2 elimination products (CH4)Ti(CH3)2+, Ti(CH3)2+, and (CH4)2Ti(C2H4)+. The binding energies for the Ti(CH4)n+ clusters were measured to be 16.8 ± 0.6, 17.4 ± 0.6, 6.6 ± 1.5, 9.8 ± 0.8, 5.1 ± 0.7 kcal/mol for n = 1–5, respectively. From analysis of the association entropies it was clear that the first solvation shell was completed at n = 4 and the fifth CH4 ligand began the second shell. For the addition of the third methane ligand to Ti+, we observed σ-bond activation to be competitive with adduct formation and dehydrogenation of the cluster produced (CH4)Ti(CH3)2+. Theoretically we characterized the Ti(CH4)n+ clusters (n = 1–3) and reproduced the trend in binding energies observed experimentally. We also calculated many local minima and several transition states on the potential energy surfaces for dehydrogenation for n = 1–3. In agreement with experiment, we found dehydrogenation of the first methane to be highly unfavorable, dehydrogenation of the second to be slightly unfavorable, and dehydrogenation of the third to be slightly favorable under the given conditions. Moreover, addition of a fourth methane resulted in further dehydrogenation and formation of an ethylene ligand bound to the metal center, (CH4)2Ti(C2H4)+. Hence, it appears that methane can be converted to ethylene in a cluster mediated σ-bond activation mechanism using first row transition metal centers at thermal energies.

Comment on “the origin of anomalous bond dissociation energies of V+(H2)n clusters”

Abstract In a recent study, we reported experimental determinations of bond dissociation energies and entropies for V + (H 2 ) n ( n = 1 to 6) and suggested that a spin change from quintet to triplet occurs upon addition of the sixth H 2 ligand. A very recent theoretical paper by Niu et al. disputes this and concludes that the experimental results are consistent with purely quintet clusters (J. Niu, B.K. Rao, S.N. Khanna and P. Jena, Chem. Phys. Letters 230 (1994) 299). In this Comment, we restate our reasons for the proposed spin charge and argue that important aspects of the results of Niu et al. are incorrect.