Effect of Noncovalent Interactions on the n-Butylbenzene···Ar Cluster Studied by Mass Analyzed Threshold Ionization Spectroscopy and ab initio Computations

Abstract

Clusters of Ar bound to isomers of the aromatic hydrocarbon n-butylbenzene (BB) have been studied using two-color REMPI (resonance enhanced multiphoton ionization) and MATI (mass analyzed threshold ionization) spectroscopy to explore noncovalent vdW interactions between these two moieties. Blue shifts of excitation energy were observed for gauche-BB • • • Ar clusters, and red shifts for anti-BB • • • Ar clusters were observed. Adiabatic ionization energies (IEs) of the conformer BB-I • • • Ar and BB-V • • • Ar were determined as 70052 and 69845 ( 5 cm -1 , respectively. Spectral features and vibrational modes were interpreted with the aid of UMP2/cc-pVDZ ab initio calculations. Data of complexation shifts of the alkyl-benzenes and their argon clusters were collected and discussed. Using the CCSD(T) method at complete basis set (CBS) level, interaction energies for the neutral ground states of BB-I • • • Ar and BB-V • • • Ar were obtained as 650 and 558 cm -1 , respectively. Combining the CBS calculation results and the REMPI and MATI spectra allowed further the determination of the interaction energies and the energetics of BB • • • Ar in the excited neutral S 1 and the D 0 cationic ground states.

Effect of Noncovalent Interactions on Conformers of the n-Butylbenzene Monomer Studied by Mass Analyzed Threshold Ionization Spectroscopy and Basis-set Convergent ab initio Computations

“…This approach is controlled by the forces between them or the molecular interactions between the moieties, known as noncovalent interactions. – These noncovalent interactions are important not only as a precursor to a chemical reaction between species but also with respect to the formation of molecular clusters about one another or as influencing factors in the structure within molecular monomer. – In this work, n -butylbenzene (BB) is investigated to understand the impact of noncovalent interactions on the energetics of its conformers. This discussion is continued for its rare gas cluster in the following paper …”

“…The values are in cm −1 . (a) is calculated at the MP2/CBS(aTZ→aQZ) level of theory, (b) is from REMPI spectra at ref , (c) is calculated from (a) and (b), (d) is from MATI spectra at ref , (e) is from MATI spectra in this work, (f) is ionic state energy resulting from the calculated S 0 energy (a) + measured two-photon transition energies (b, c, d) and (g) is the calculated ionic stated energy from (a) and the UMP2/cc-pVDZ level of theory for D 0 state.…”

“…This discussion is continued for its rare gas cluster in the following paper. 13 In our previous study, the S 1 r S 0 and D 0 r S 1 transitions of two conformers of BB (neutral ground state, S 0 ; neutral excited state, S 1 ; cationic ground state, D 0 ) were studied in a skimmed supersonic jet expansion with REMPI (resonance enhanced multiphoton ionization) and MATI (mass analyzed threshold ionization) spectroscopy. 14 The spectra obtained were discussed in the context of conformer specific ionization-induced geometry changes, and complementary subsequent work relating to partially rotationally resolved zero electron kinetic energy (ZEKE) spectra was analyzed with a spectator orbital model.…”