Article pubs.acs.org/JPCA
Valence and Ionic Lowest-Lying Electronic States of Isobutyl Formate Studied by High-Resolution Vacuum Ultraviolet Photoabsorption, Photoelectron Spectroscopy, and Ab Initio Calculations ,†,‡ ́ M. A. Smiałek,* M. Łabuda,§ J. Guthmuller,§ S. V. Hoffmann,∥ N. C. Jones,∥ M. A. MacDonald,⊥ L. Zuin,⊥ N. J. Mason,‡ and P. Limaõ -Vieira*,#,‡ †
Department of Control and Power Engineering, Faculty of Ocean Engineering and Ship Technology, Gdańsk University of Technology, Gabriela Narutowicza 11/12, 80-233 Gdańsk, Poland ‡ Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom § Department of Theoretical Physics and Quantum Information, Faculty of Applied Physics and Mathematics, Gdańsk University of Technology, Gabriela Narutowicza 11/12, 80-233 Gdańsk, Poland ∥ ISA, Department of Physics and Astronomy, Aarhus University, DK 8000 Aarhus C, Denmark ⊥ Canadian Light Source Inc., 44 Innovation Boulevard, Saskatoon, Saskatchewan S7N 2V3, Canada # Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal S Supporting Information *
ABSTRACT: The highest resolution vacuum ultraviolet photoabsorption spectrum of isobutyl formate, C5H10O2, yet reported is presented over the energy range 4.5−10.7 eV (275.5−118.0 nm) revealing several new spectral features. Valence and Rydberg transitions and their associated vibronic series observed in the photoabsorption spectrum have been assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. Calculations have also been carried out to determine the ionization energies and fine structure of the lowest ionic state of isobutyl formate and are compared with a newly recorded photoelectron spectrum (from 9.0 to 27.0 eV). The value of the first ionization energy was determined to be 10.508 eV (adiabatic) and 10.837 eV (vertical). New vibrational structure is observed in the first photoelectron band, predominantly resulting from C−O and CO stretches of the molecule. The photoabsorption cross sections have been used to calculate the photolysis lifetime of isobutyl formate in the upper stratosphere (20−50 km), indicating that the hydroxyl radical processes will be the main loss process for isobutyl formate.
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INTRODUCTION Isobutyl formate, C5H10O2, also known as 2-methylpropyl methanoate, belongs to the aliphatic esters and is derived from a reaction of isobutanol with formic acid. Because of its fruity (raspberry) aroma, this compound is often used in food flavoring. It has also been found in all varieties of alcoholic beverages.1−4 Basic physical properties of isobutyl formate, including boiling point, density, viscosity, vapor pressure values of critical pressure and temperature, specific inductive capacity, and refractive index are known.5 The occurrence of esters in the atmosphere is not only due to industrial processes, they are also emitted from biogenic sources or generated through oxidation of other volatile compounds. It has been shown that esters may be a product of hydroxyl radicalinitiated oxidation of various ethers in troposphere.6 Although esters are not very toxic, their degradation in the atmosphere may lead to the production of more toxic and reactive species. © 2015 American Chemical Society
The importance of this compound is in its industrial application. Isobutyl formate was found to be a relatively good absorbent for CO2, although its solubility performance is worse than for other formates with longer and linear side chains (for example, butyl formate). This is important in precombustion CO2 capture in coal-fired power plants.7 Isobutyl formate undergoes Norrish Type II elimination, leading to the formation of relatively high yields of formic acid at 5.4 and 5.6 eV incident radiation energy,8 that upon further photolysis may lead to CO, CO2, and free radical formation.9 Isobutyl formate has also been found as a trace constituent of Los Angeles smog.10 Although this compound has been in use for many years, and its presence in the atmosphere has been documented, very little Received: June 24, 2015 Revised: July 15, 2015 Published: July 15, 2015 8647
DOI: 10.1021/acs.jpca.5b06053 J. Phys. Chem. A 2015, 119, 8647−8656
Article
The Journal of Physical Chemistry A information on the structure of this molecule can be found in the literature. The very first infrared (IR) spectroscopic studies revealed the vibrational structure of isobutyl formate.11 A doubly split band was observed corresponding to CH3 deformation, resulting from the methyl groups attached to the same carbon at 1370 and 1390 cm−1. Also a split band, corresponding to CH2 group deformation, was identified at 1460 cm−1, along with a band arising from the isopropyl group at 760 cm−1. A band visible at 1185 cm−1 was attributed to C−O vibration coupled with a vibration of C−C. Some later IR studies also revealed the presence of a pronounced band corresponding to the combination of formyl C−H (2937 cm−1) and CO (1730 cm−1) stretching vibrations, which appears at 4667 cm−1.12 Development of NMR techniques and attempting to describe some conformational behavior of this ester allowed a conclusion that only two stable planar conformations about acyl−oxygen bond are possible.13 In the case of isobutyl formate, where the optimum conformation requires two gauche interactions, the value of the proton−proton coupling constant was reduced in comparison to other formate esters that were the subject of this study. Additional studies performed with the assistance of lowresolution microwave spectroscopy showed that for isobutyl formate an extended conformational form coupled with gauche configuration can be assigned.14 In this paper, we present high-resolution vacuum ultraviolet (VUV) photoabsorption spectra with absolute cross sections, together with theoretical calculations of the vertical excitation energies and oscillator strengths of the electronic transitions of isobutyl formate. To our knowledge, no high-resolution studies have been reported for this molecule yet. Additionally, the photoelectron spectrum (PES) was measured and analyzed in order to clarify Rydberg assignments in the VUV photoabsorption spectrum. These data also provide for the first time a vibrational resolution of the first ionic band of isobutyl formate. Absolute photoabsorption cross sections are needed in modeling studies of the Earth’s atmosphere and radiation chemistry of aliphatic esters. Together with previously analyzed data for methyl formate15 and ethyl formate,16 the detailed analysis of electronic structure of this molecule will help to understand the chemistry of esters.
Figure 1. High-resolution photoabsorption spectrum of isobutyl formate, C5H10O2.
Figure 2. Photoelectron spectrum of isobutyl formate, C5H10O2 in the 10.0−27 eV region (*indicates water contamination) at the incident photon energy hν = 80 eV and assignment of the IEs.
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