A study of nonrigid aromatic molecules. Observation and spectroscopic analysis of the stable conformations of various alkylbenzenes by supersonic molecular jet laser spectroscopy

Abstract

The technique of supersonic molecular jet laser spectroscopy was used to determine the stable conformations of a series of alkylbenzenes. This study demonstrates, for the first time, the sensitivity of molecular jet spectroscopy in determining both the number of stable conformations as well as the geometry of various ethyl, propyl, and butyl substituents relative to the aromatic ring. Different rotamers with low barriers to interconversion, <5 kcal/mol, can be isolated in the supersonic jet expansion. Each observed conformation exhibits its own spectroscopic origin (S[ *-S0 transition) in a two-color time-of-flight mass spectrum (TOFMS). The number of stable conformations is then used to determine the minimum energy geometries of the substituent group. Previous identification of individual molecular conformations for such low barriers to interconversion has not been attainable with conventional techniques such as variable-temperature NMR.

Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of the 35Cl and 37Cl isotopomers of p-chloroanisole

“…Due to the natural abundance of chlorine isotopes, the 35 Cl and 37 Cl isotopomers of p-chloroanisole were detected by high-resolution time-of-flight (TOF) mass spectrometry, and thus, the resonance-enhanced two-photon ionization (R2PI) spectra [12,13] of the two isotopomers were measured. Because the R2PI process occurs when the laser wavelength is tuned to an intermediate state of the molecule, the R2PI spectrum of pchloroanisole not only provides useful information about the molecular properties, but also presents a promising method for the determination of p-chloroanisole [14,15]. Calculations by ab initio and density functional theory (DFT) methods were performed to predict the molecular geometry of p-chloroanisole and to interpret the experimental results.…”

Solid state proton spin relaxation in ethylbenzenes: Methyl reorientation barriers and molecular structure

“…One strength of the SSPSR technique lies in the fact that barriers down to about 3 kJ/mol can be measured above liquid nitrogen temperatures (77 K) and considerably lower barriers down to about 0.5 kJ/mol can be measured at lower temperatures. 4 In SSPSR studies, it is nuclear magnetic reso- nance activation energies that are measured, but in most models these can usually be identified with reorientation barriers to within reasonable accuracy.5,6 Barriers below about 20 kJ/mol are difficult to study by dilute liquid high resolution nuclear magnetic resonance (NMR) spectroscopy.7-9 From the measurement of a sufficient number of appropriate J couplings in high resolution NMR spectra, barriers in the range of 0.8-13 kJ/mol can be determined in very limited cases.1O These cases do not include threefold rotational barriers for methyl groups. Inelastic neutron scattering techniques can determine barriers over a wide range at low temperatures in the solid state where tunneling rather than thermally activated rotation is the dominant reorientation process.…”

Gas phase conformation in the ibuprofen analogues isobutylbenzene and 2-phenylpropionic acid

“…Studying these side chains in isolation greatly simplifies the task of sorting out conformational preferences. The R2PI spectrum of IBB reported by Seeman et al is dominated by a strong peak in the S 1 -S 0 origin region [17]. A single, non-symmetric AG conformer (A and G refer to anti and gauche arrangement of the two methyl groups with respect to the C -C bond) was inferred, as meta-substitution of the ring leads to two strong origin bands.…”