Barrierless Proton Transfer in the Weak C–H···O Hydrogen Bonded Methacrolein Dimer upon Nonresonant Multiphoton Ionization in the Gas Phase

Abstract

Intermolecular proton transfer (IMPT) in a C–H···O hydrogen bonded dimer of an α,β-unsaturated aldehyde, methacrolein (MC), upon nonresonant multiphoton ionization by 532 nm laser pulses (10 ns), has been investigated using time-of-flight (TOF) mass spectrometry under supersonic cooling condition. The mass peaks corresponding to both the protonated molecular ion [(MC)­H+] and intact dimer cation [(MC)2]•+ show up in the mass spectra, and the peak intensity of the former increases proportionately with the latter with betterment of the jet cooling conditions. The observations indicate that [(MC)2]•+ is the likely precursor of (MC)­H+ and, on the basis of electronic structure calculations, IMPT in the dimer cation has been shown to be the key reaction for formation of the latter. Laser power dependences of ion yields indicate that at this wavelength the dimer is photoionized by means of 4-photon absorption process, and the total 4-photon energy is nearly the same as the predicted vertical ionization energy of the dimer. Electronic structure calculations reveal that the optimized structures of [(MC)2]•+ correspond to a proton transferred configuration wherein the aldehydic hydrogen is completely shifted to the carbonyl oxygen of the neighboring moiety. Potential energy scans along the C–H···O coordinate also show that the IMPT in [(MC)2]•+ is a barrierless process.

Factors contributing to halogen bond strength and stretch or contraction of internal covalent bond

“…[1][2][3][4][5][6] The factors that contribute to its strength have been elucidated, as well as secondary issues such as the circumstances which propel the bridging proton to transfer from one subunit to the other. [7][8][9][10][11][12][13][14] An enormous body of work has developed the ability to estimate the strength of a given HB based upon spectroscopic data such as the downfield shift of the NMR signal of the central proton. 15,16 Another trademark of a AHÁ Á ÁB H-bond is the bathochromic shift of the A-H stretching frequency, coupled with the intensification of this band in the IR spectrum.…”

Enhancement of tetrel bond involving tetrazole-TtR3(Tt = C, Si; R = H, F). Promotion of SiR3transfer by a triel bond

“…While decades of study of the proton transfer process have led to a well understood set of circumstances that will lead to such a transfer, [30][31][32][33][34][35][36][37][38] examination of tetrel transfer remains in its infancy. A central question concerns the conditions under which a TtR 3 group can be promoted to transfer from one molecule to another.…”

Ultrafast Protonation of an Amide: Photoionization-Induced Proton Transfer in Phenol-Dimethylformamide Complex Cation

“…Our goal is to elucidate how the system evolves in the ionic collision complex from an initial nonproton-transferred (non-PT) to a final proton-transferred (PT) configuration, before the complex exits for the product channel. In spite of recent research interests on photoionization-induced PT in molecular complexes, − ultrafast time-resolved measurements of such reactions are still scarce. , …”