A spectroscopic and computer simulation study of butanol vapors

Abstract

Clusters of butanol formed above neat liquid samples were entrained in a supersonic jet and probed using 10.5 eV vacuum ultraviolet laser single-photon ionization/time-of-flight mass spectrometry. The four different isomers of butanol (n-butanol, sec-butanol, iso-butanol, and tert-butanol) were studied separately to assess the influence of the structure of the alkyl chain on the formation and stability of the hydrogen bonded clusters. Most of the higher mass features observed in the mass spectra could be assigned to protonated alcohol clusters, H(ROH)n+, n⩽3; R=C4H9, that arise from facile proton-alkoxy radical/alkoxide anion dissociation. Signals due to protonated trimers were only evident in the spectra of tert- and sec-butanol. Empirical force fields, density functional theory and ab initio methods were used to identify the geometries of all clusters up to the pentamers for the different isomers. Monte Carlo simulations established vapor-phase cluster distributions, while molecular dynamics was used to assess the relative stability of the isomeric tetramers. Together, these experimental and theoretical results suggest that butanol tetramers are “magic-number” structures, and that the protonated ion signals of size n could be correlated with the neutral cluster of size n+1, provided the vapor pressures sampled in the supersonic jet exceeded equilibrium values.

Geometries and energetics of methanol–ethanol clusters: a VUV laser/time-of-flight mass spectrometry and density functional theory study

“…The interplay between the formation of H-bonding and the steric repulsion between alkyl groups determine the structure and stability of alcohol clusters. For small alcohol clusters such as methanol (Me) or ethanol (Et), H-bonds play a key role in determining the most-stable structures (4), whereas the alkyl chains have a more pronounced effect in larger clusters, as shown in our earlier studies of butanols (5). Despite the extensive amount of research that has been done on neat alcohols (1)(2)(3), the number of studies on alcohol mixtures, specifically on their vapor cluster compositions, is quite limited.…”

Prediction of the Size Distributions of Methanol−Ethanol Clusters Detected in VUV Laser/Time-of-Flight Mass Spectrometry

“…This indicates that supersaturation plays a crucial role in the cluster size distributions for vapor mixtures, as it was found for neat alcohols. 1,2 Appendix A discusses the effect of fugacity or pressure on cluster size distributions. The probability-fugacity relation given by eq A.1 of Appendix A permits the cluster size distribution at any fugacity to be computed from a known distribution.…”

“…We then obtained the ionized cluster distributions by adding the probabilities of all ionized clusters of the same size where P ion (m,n) is a the probability of forming an ionized cluster Me m Et n H + , whereas P neu (m,n) is that of a neutral cluster Me m Et n . For example, the probability of an ionized cluster Me 2 Et 1 H + [P ion (2,1)] is a weighted sum of the probabilities of its neutral precursor clusters, Me 3 Et 1 and Me 2 Et 2 [(3/4)P neu (3,1) + (1/ 2)P neu (2,2)]. …”

“…More detailed descriptions of the experimental apparatus and procedures can be found elsewhere. [1][2][3][4] It has been established for alcohols that protonated N-mers are predominately formed by the single-photon ionization of neutral (N + 1)-mer clusters. [1][2][3]5 In addition, pressure studies support our assertion that the neutral cluster distributions in the vacuum chamber prior to ionization reflect the equilibrium cluster distributions above liquid mixtures.…”

“…1,2 However, the exact level of supersaturation that leads to the experimentally observed cluster distribution has not been established quantitatively. The role of temperature in the cluster size distribution is not yet well understood.…”