Electronic State Spectroscopy of Halothane As Studied by ab Initio

Article pubs.acs.org/JPCA

Electronic State Spectroscopy of Halothane As Studied by ab Initio Calculations, Vacuum Ultraviolet Synchrotron Radiation, and Electron Scattering Methods F. Ferreira da Silva,† D. Duflot,‡ S. V. Hoffmann,§ N. C. Jones,§ F. N. Rodrigues,∥,⊥ A. M. Ferreira-Rodrigues,∥,# G. G. B. de Souza,∥ N. J. Mason,∇ S. Eden,∇ and P. Limaõ -Vieira*,†,∇ †

Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal ‡ Laboratoire de Physique des Lasers, Atomes et Molécules (PhLAM), UMR CNRS 8523, Université de Lille, F-59655 Villeneuve d’ Ascq Cedex, France § ISA, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark ∥ Instituto de Química, Universidade Federal do Rio de Janeiro, Ilha do Fundão, 21949-900 Rio de Janeiro, RJ, Brazil ⊥ Departamento da Ciência da Natureza e Matemática, Instituto Federal de Educaçaõ , Ciência e Tecnologia do Rio de Janeiro, Maracanã, 20270-021 Rio de Janeiro, RJ, Brazil # DCN, Instituto de Biociências, Universidade Federal do Estado do Rio de Janeiro, Urca, 22290-240 Rio de Janeiro, RJ, Brazil ∇ Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, U.K. S Supporting Information *

ABSTRACT: We present the first set of ab initio calculations (vertical energies and oscillator strengths) of the valence and Rydberg transitions of the anaesthetic compound halothane (CF3CHBrCl). These results are complemented by high-resolution vacuum ultraviolet photoabsorption measurements over the wavelength range 115−310 nm (10.8−4.0 eV). The spectrum reveals several new features that were not previously reported in the literature. Spin−orbit effects have been considered in the calculations for the lowest-lying states, allowing us to explain the broad nature of the 6.1 and 7.5 eV absorption bands assigned to σ*(C−Br) ← nBr and σ*(C−Cl) ← nCl transitions. Novel absolute photoabsorption cross sections from electron scattering data were derived in the 4.0−40.0 eV range. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of halothane in the upper stratosphere (20−50 km). lower atmosphere.5 Therefore, the earth’s radiative balance may be disturbed by greenhouse heating that depends on their atmospheric lifetimes. It is also important to assess the photolysis of these molecules as a potential source of chlorine and bromine radicals that can contribute to stratospheric ozone depletion.6 Brown and co-workers6 have measured the reactions of halothane with hydroxyl (OH) radicals to show that this is likely to be the main sink mechanism for these molecules in the troposphere, with a lifetime of the order of 2 years. Meanwhile, Langbein et al.1 measured the room-temperature rate coefficient for reactions of the OH radical with halothane (k = 1.5 × 10−14 cm3 molecules−1 s−1) and used this to calculate the atmospheric lifetime τOH = 7.0 yr. This indicates that surface emissions of halothane can easily reach the stratosphere. Langbein et al.1 obtained a value of ∼100 yr for the photochemical lifetime of halothane at an altitude of 36 km, whereas the global warming potential relative to CFC-12 yield

1. INTRODUCTION The present work on halothane, CF3CHBrCl, is part of a wider research program aimed at understanding the spectroscopy of halogen-containing species and the role of these trace gases in atmospheric chemistry and physics. Our knowledge of the vacuum ultraviolet (VUV) electronic state spectroscopy of anaesthetics remains poorly quantified in a wide wavelength region. Indeed, recent experimental information on such chemicals is mainly restricted to λ > 190 nm (1.5 eV) when compared to EOM-CCSD calculations. Thus, the active space was augmented with one occupied orbital (10 electrons, 12 MOs). This space was used as the reference for an internally contracted MRCI evaluation of spin−orbit corrected transition energies and oscillator strengths.

2. BRIEF SUMMARY OF THE STRUCTURE AND PROPERTIES OF HALOTHANE Theoretical calculations show that the staggered conformer of CF3CHBrCl is more stable than the eclipsed conformer.7,8 The vertical energies of the lowest ionic states of halothane (ground and first excited) have been determined by Dumas et al.4 using He(I) photoelectron spectroscopy. Recent photodissociation dynamical studies have been carried out on the σ*(C−Br) ← nBr transition using resonance-enhanced multiphoton ionization time-of-flight (REMPI-TOF) mass spectrometry9 as well as photofragmentation of core-excited halothane near the C 1s ionization edge.10 Halothane has symmetry C1 in the electronic ground state. The symmetry class available to a C1 molecule is A, and the calculated electron configuration of the outermost valence orbitals of the X 1A ground state is ···(41a)2 (42a)2 (43a)2 (44a)2 (45a)2 (46a)2. The highest occupied molecular orbital (HOMO) and the second highest occupied molecular orbital (HOMO−1) have Br 4p lone pair character, whereas the HOMO−2 (44a) has Cl 3p lone pair character. The lowest unoccupied molecular orbitals, 47a (LUMO) and 48a (LUMO +1), are mainly of σ*(C−Br) and σ*(C−Cl) antibonding character. These are shown as Supporting Information data. The earlier low-resolution studies in ref 4 have shown that the lowest Rydberg states may overlap with valence states, resulting in a complex intensity distribution in the electronic spectrum. It is therefore necessary to separate features in the photoabsorption spectra due to Rydberg states from those arising from valence states. The former states may be identified through knowledge of the ionization states and the application of quantum defect theory. The four lowest vertical ionization energies, which are needed to calculate the quantum defects associated with transitions to Rydberg orbitals, have been

4. EXPERIMENTAL DETAILS 4.1. VUV Photoabsorption. The high-resolution VUV photoabsorption spectrum of halothane was recorded using the UV1 beamline of the ASTRID synchrotron facility at the the Aarhus University, Denmark (Figure 1). The experimental apparatus has been described in detail elsewhere;24 therefore, only a brief review will be given here. Synchrotron radiation passes through a static gas sample, and a photomultiplier is used to measure the transmitted light intensity. The incident wavelength is selected using a toroidal dispersion grating with 2000 lines/mm providing a resolution of 0.075 nm, corresponding to 3 meV at the midpoint of the energy range studied. For wavelengths below 200 nm (energies above 6.20 eV), helium was flushed through the small gap between the photomultiplier and the exit window of the gas cell to prevent any absorption by molecular oxygen in the air contributing to the spectrum. The sample pressure is measured using a capacitance manometer (Baratron). To ensure that the data is 8504

DOI: 10.1021/acs.jpca.5b05308 J. Phys. Chem. A 2015, 119, 8503−8511

Article

36.15 19.33 5s

5p 5p

5s/σ*(C−Cl)

Mean value of r2 (electronic radial spatial extents). bThe last decimal of the energy value is given in brackets for these less-resolved features. a

HOMO → 5p + HOMO−3 → 5s

σ*(C−Cl) σ*(C−Cl) σ*(C−Br)

− 0.0063 0.0031 0.0132 0.0176 0.0546 0.0266 0.0215 0.0164 0.0007 0.2020 0.0170 0.0529 0.0379 0.0667 − 6.103 6.149 7.406 7.478 8.099 8.175 8.190 8.367 8.631 8.918 9.064 9.153 9.303 9.367

129 133 133 133 133 149 153 143 142 137 140 157 159 157 160

σ*(C−Br)

free of any saturation effects, the absorption cross sections were measured over the pressure range of 0.03−0.68 Torr, with typical attenuations of less than 40%. The synchrotron beam ring current is monitored throughout the collection of each spectrum, and background scans are recorded with the cell evacuated. Absolute photoabsorption cross sections are then obtained using the Beer−Lambert attenuation law: It = I0 exp(−nσx), where It is the radiation intensity transmitted through the gas sample, I0 is that through the evacuated cell, n the molecular number density of the sample gas, σ the absolute photoabsorption cross section, and x the absorption path length (25 cm). The accuracy of the cross section is estimated to be ±5%. Only when absorption by the sample is very weak (I0 ≈ It) does the error increase as a percentage of the measured cross section. 4.2. Electron Scattering. Whereas the present photoabsorption experiments yield absolute cross sections, the essential parameter in electron impact measurements is the generalized oscillator strength, f n(K). This is related to the differential inelastic electron impact cross section [∂2σ/∂E∂Ω] by f n(K) = (E/2)(k0/kn)K2[∂2σ/∂E∂Ω], where E is the electron energy loss and k0 and kn are the incident and scattered electron momenta; K is the momentum transfer, and Ω is the scattering solid angle. The experimental electron scattering data is obtained by measuring in the discrete as well as in the continuum regions the differential (i.e., df/dE) oscillator strengths. As far as the discrete region is concerned, this is due to the finite line width and spectrometer bandwidth, while in the continuum, this is due to the continuous nature of the final states. Under experimental conditions in which the momentum transfer is negligible (high incident electron energy, near zero degree scattering angle), dipole selection rules apply and the generalized oscillator strength becomes identical to the optical oscillator strength, f n0(E). Hence, the differential generalized oscillator strength becomes identical to the differential optical oscillator strength.25 The first step is to get the relative intensity of the generalized oscillator strength by running at an intermediate impact energy, variable-angle, low-resolution spectrometer, which has been described elsewhere.26,27 Briefly, the Rio de Janeiro spectrometer consists of an electron gun, a gas inlet system at 90° with respect to the electron beam, a Möllenstedt electron analyzer, and a conventional detection system. The electron gun, set at 1 keV, can be rotated from −60° to +60°. A 2.0° energy-loss spectrum was obtained for halothane in the 4.0−40.0 eV energy-loss range. The energy resolution was 0.8 eV. The halothane spectrum is converted into relative differential generalized strength and then extrapolated to the optical limit using the universal formula of Msezane and Sakmar.28 This extrapolation procedure has been discussed in detail elsewhere.29 The procedure to obtain absolute cross section values has also been described previously in some detail.30,31 Briefly, the absolute values for the differential oscillator strength spectrum are obtained by applying the S(−2) sum rule:25,32 S(−2) = ∫ R(E/EH)−2 (df/dE) dE = α where EH is the Hartree energy constant (27.21 eV) and α is the halothane static electric-dipole polarizability, α = 9.37 Å3.4 In the sum rule normalization procedure, the contribution for the photoabsorption above the upper limit of the measured spectrum (40 eV) can be estimated by fitting a polynomial of the form df/dE = AE−2 + BE−3 + CE−4 to the smooth continuum of the relative photoabsorption data and extrapolating to infinite energy. From the relative optical oscillator strength (or more

9.040 9.329

34.29 n(Br)/n(Cl) → 5s HOMO−2 → 5s/ σ*(C−Cl) + HOMO−1 → σ*(C−Br) n(Br)/n(Cl) → σ*(C−Cl) n(Br)/n(Cl) → σ*(C−Cl) n(Br)/n(Cl) → σ*(C−Cl)

8.926

8.26 13.7(4) 22.89 7.54(2) 7.84(2) 8.12(5)

1.28 6.13(2)

cross section (Mb) exptl. (eV)b mixed character HOMO−3 (43a) HOMO−2 (44a) HOMO−1 (45a) HOMO (46a) ⟨r2⟩a fL E (eV)

state X̃ 1A Ã 1A B̃ 1A C̃ 1A D̃ 1A Ẽ 1A F̃ 1A G̃ 1A H̃ 1A I ̃ 1A J ̃ 1A K̃ 1A L̃ 1A M̃ 1A Ñ 1A

Table 1. Calculated Vertical Excitation Energies (EOM-CCSD/aug-cc-pVTZ + Rydberg Level, CCSD/(PP)-cc-pVTZ Geometry) (Electronvolts) and Oscillator Strengths Compared with the Present Experimental VUV Absorption Cross Sections of Halothane, CF3CHBrCl

The Journal of Physical Chemistry A

8505

DOI: 10.1021/acs.jpca.5b05308 J. Phys. Chem. A 2015, 119, 8503−8511

Article

The Journal of Physical Chemistry A

Table 2. Calculated Vertical Excitation Energies (EOM-CCSD/cc-pVTZ, CCSD/(PP)-cc-pVTZ Geometry) (Electronvolts) and Oscillator Strengths for the Lowest-Lying Singlet and Triplet States of Halothane, CF3CHBrCl, without and with Spin−Orbit Effects no spin−orbit E (eV)

CAS(10,12)/MRCI X̃ 1A

a

ã 3A

n(Br) (HOMO) → σ*(C−Br)

0.000 5.783

b̃ 3A

n(Br) (SHOMO) → σ*(C−Br)

5.899

à 1A B̃ 1A c̃ 3A

n(Br) (HOMO) → σ*(C−Br) n(Br) (SHOMO) → σ*(C−Br) n(Cl) (HOMO−2) → σ*(C−Cl)

6.515 6.585 6.903

d̃ 3A

n(Cl) (HOMO−3) → σ*(C−Cl)

7.115

C̃ 1A D̃ 1A

n(Cl) (HOMO−2) → σ*(C−Cl) n(Cl) (HOMO−3) → σ*(C−Cl)

7.978 8.070

with spin−orbit

fL

0.0055 0.0028

0.0171 0.0172

E (eV)

fL

0.000 5.682 5.686 5.760 5.865 5.994 6.013 6.537 6.597 6.914 6.920 6.923 7.121 7.122 7.123 7.985 8.072