Fourier Transform Microwave and Millimeter-Wave Spectroscopy of

Article pubs.acs.org/JPCA

Fourier Transform Microwave and Millimeter-Wave Spectroscopy of Bromoiodomethane, CH2BrI S. Bailleux,*,† D. Duflot,† K. Taniguchi,‡ S. Sakai,‡ H. Ozeki,‡ T. Okabayashi,§ and W. C. Bailey∥ †

Laboratoire de Physique des Lasers, Atomes et Molécules, UMR CNRS 8523, Université de Lille, F-59655 Villeneuve d’Ascq Cedex, France ‡ Department of Environmental Science, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi 274-8510, Japan § Graduate School of Science and Technology, Shizuoka University, Oya 836, Surugaku, Shizuoka 422-8529, Japan ∥ Chemistry-Physics Department, Kean University, Union, New Jersey 07083, United States S Supporting Information *

ABSTRACT: Bromoiodomethane, CH2BrI, is a molecule of natural origin emitted in significant amount into the marine boundary layer. It can easily be decomposed by solar radiation, releasing Br and I atoms in the troposphere, which in turn impacts the atmospheric chemistry. Spectroscopy is an invaluable tool to monitor species present in the atmosphere. Since no high-resolution spectroscopic studies are available for this dihalomethane, we have investigated the rotational spectra of the two bromine isotopologues of CH2BrI in its vibrational ground state in the microwave and millimeter-wave regions. Transitions of b-type have been recorded by Fourier transform microwave spectroscopy below 25 GHz while both a- and b-type spectral lines have been measured below 230 GHz. Observed transitions correspond to energy levels with J ≤ 132 and Ka ≤ 14. Molecular constants including those describing the nuclear quadrupole coupling tensors for 79Br, 81Br, and 127I were accurately determined from the least-squares analysis of a total of 1873 distinct transition frequencies (of which 943 belong to the CH279BrI isotopologue). An experimental (r0) structure of the title species has been derived from the two sets of rotational constants.

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INTRODUCTION Bromoiodomethane, CH2BrI, is known to be of atmospheric relevance. No significant anthropogenic origins for this compound are known, and oceans are the primary natural sources of bromoiodomethane and other iodocarbons such as CH3I, CH2I2, CH2ICl, and C2H5I in the Earth’s atmosphere. Among these organic iodine species, those containing two chromophores (CH2ClI, CH2BrI, and CH2I2) are the most photolabilethey undergo a rapid solar photolysis to produce a halomethyl radical and a reactive iodine atom.1 With a tropospheric lifetime ranging from a few days (CH3I) to a few minutes (CH2I2), these dihalomethanes are denominated as very short-lived halocarbons. That of CH2BrI is estimated to be of the order of 2.5 h.2 It can therefore be considered that such iodomethanes are of importance mainly for the tropospheric chemistry. Although iodocarbons are found at lower levels than their brominated counterparts, it has been demonstrated that the combined effects of reactive iodine and bromine chemistry affect the oxidative capacity of the troposphere (this selfcleaning process controls the lifetime of many trace gases) more effectively.2,3 This occurs through efficient tropospheric ozone depleting cycles, changes of the partitioning of important species like HOx and NOx (x = 1, 2), and increased rates of dimethyl sulfide oxidation, to name a few.2 In addition, iodine is involved in new particle formation, which may grow to become © 2014 American Chemical Society

cloud condensation nuclei, thus impacting the radiative balance of the atmosphere and hence the climate.4,5 Remote sensing using microwave and (sub)millimeter-wave heterodyne spectroscopy from orbiting satellites is a powerful tool that can provide precise measurements of atmospheric trace species abundances from their thermal emission spectra.6−9 This technique has a long history and is capable of monitoring changes and studying processes in the Earth’s atmosphere, from the troposphere to the mesosphere. Preliminary spectroscopic investigation of CH2BrI in the laboratory is thus essential if one desires to monitor this species in the troposphere. No experimental spectroscopic studies of the CH2BrI molecule have so far been reported. As far as high-resolution microwave/millimeter-wave spectroscopy is concerned, several reasons for this can be invoked, all lying in the difficulties in identifying and assigning the rotational spectrum. These include (i) the complex electric quadrupole patterns due to the presence of two different nuclei with large nuclear spin (IBr = 3/2 and II = 5/2) implying that each rotational level (except for J < 4) is split into 24 hyperfine sublevels, (ii) the fact that Received: October 7, 2014 Revised: November 19, 2014 Published: November 24, 2014 11744

dx.doi.org/10.1021/jp510119e | J. Phys. Chem. A 2014, 118, 11744−11750

The Journal of Physical Chemistry A

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species of CH2BrI to assist with assignment of the experimental spectra. These are the rotational constants, inertial axes components of the 79Br, 81Br, and 127I nuclear quadrupole coupling constant (NQCC) tensors, and centrifugal distortion constants (CDC) - quartic and sextic; as well as electric dipole moment components for prediction of the relative intensity of rotational transitions. All calculations were made with the Gaussian 03 suite of programs.18 Molecular Structure. Calculation of the spectroscopic constants requires, of course, a molecular structure on which to make the calculations. Thus, the structure was optimized at the CCSD(T)/(aug)-cc-pVTZ-PP and MP2/SDB-VTZ levels of theory. CCSD(T) is coupled-cluster theory with single, double, and iterative triple excitations,19,20 (aug)-cc-pVTZ-PP is an abbreviation used here for aug-cc-pVTZ bases21 on C and H, and cc-pVTZ-PP on Br and I. PP stands for pseudopotential, i.e., 10 and 28 core electrons were treated through a relativistic PP for bromine22 and iodine,23 respectively. MP2 is secondorder Møller−Plesset perturbation theory,24 SDB-VTZ is an abbreviation used here for Dunning cc-pVTZ bases on C and H, and SDB-cc-pVTZ25,26 on Br and I, which used 28 and 46 core electrons in the relativistic PP, respectively. Bases ccpVTZ-PP and SDB-cc-pVTZ were obtained from the EMSL27 basis set library.28,29 Molecular structure parameters thus derived are given in Table 1 along with corresponding electric dipole moment components. CH2BrI possesses a weak μa (≤0.2 D) and a larger μb (∼1.5 D) permanent dipole moment.

nuclear quadrupole coupling parameters (including the offdiagonal terms) are of similar magnitudes when compared with the value of the B and C rotational constants, (iii) high-J rotational transitions occur at low frequency as a consequence of the small value of these two rotational constants, (iv) the rich satellite structure arising from the low frequency of the ICBr bending vibrational mode (∼144 cm−1, ref 10), and (v) the overlapping of the spectra of the two Br isotopologues of CH2BrI since the natural abundances of 79Br and 81Br are nearly identical. Puzzarini et al.11 and Bailleux et al.12 have recently published the analysis of the rotational spectrum of fluoro- and chloroiodomethane, respectively, while Kisiel et al. reported that of CH2I2 more than a decade ago.13 In order to complete the study of the rotational spectra of the CH2IX dihalospecies, where X = {F, Cl, Br, I}, we have investigated those of CH279BrI and CH281BrI in the microwave and millimeter-wave regions. Assignment of the spectra was assisted by high-level quantum chemistry computations undertaken to predict relevant spectroscopic constants. Our data could be used in the future to help assign vibrational bands. Ultimately the atmospheric abundance of this important dihalogenated organic compound could also be monitored in order to better understand its implications in the ozone loss and global climate change.

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EXPERIMENTAL DETAILS The rotational spectrum of bromoiodomethane was investigated with a commercial sample (Sigma-Aldrich, 97% purity) manipulated without additional purification. We were guided in the search for the rotational spectrum of the two Br-isotopologues of CH2BrI by computed molecular constants. Since both species not only have similar rotational and centrifugal distortion constants, but have nearly identical natural abundance (79Br: 50.69%, 81Br: 49.31%), their rotational spectra are strongly blended, and selected lines were recorded. With this in mind, we first investigated the spectra in the microwave (mw) region and subsequently in the millimeterwave (mmw) region to avoid misassignments. Microwave spectra were recorded between 23.0 and 25.0 GHz with the Balle−Flygare-type Fourier-transform mw spectrometer14 at Shizuoka University.15 This frequency region corresponds to low-J, Q-branch transitions (QKa″=0(J″) with f ≤ 23.8 GHz) and to the fundamental R0(0) rotational line (JKaKc = 111 ← 000, f ∼ 25.0 GHz), both of b-type. CH2BrI was diluted in Ar at a concentration of 1% and was released into a vacuum chamber from a nozzle (General Valve Co.) at a stagnation pressure of 1.5 atm. Millimeter-wave spectra were acquired at Toho University between 122 and 175 GHz. The characteristics of the spectrometer in Toho University are detailed in ref 16. Follow-up measurements were performed at the University of Lille between 199 and 230 GHz. The spectrometer employed in Lille is thoroughly described in ref 17. Care was taken to resolve as much as possible the hyperfine structure due to both halogen nuclei by tuning the modulation depth in the 150−250 kHz range while keeping a low pressure in the absorption cell (6 mTorr).

Table 1. CCSD(T)/(aug)-cc-pVTZ-PP and MP2/SDB-VTZ Optimized Structure Parameters, CH279BrI Non-Zero Electric Dipole Moment Components parametera

CCSD(T)/(aug)-cc-pVTZ-PPb

MP2/SDB-VTZc

CBr CI CH BrCI BrCH HCH |μa| |μb|

1.9264 2.1313 1.0830 113.96 107.99 111.95 0.20 1.44

1.9197 2.1306 1.0817 114.17 108.10 111.65 0.09 1.54

a

Bond lengths in Å, bond angles in degrees. Dipole moment in D. aug-cc-pVTZ on C and H; cc-pVTZ-PP on Br and I. caug-cc-pVTZ on C and H; SDB-cc-pVTZ on Br and I. b

Nuclear Quadrupole Coupling Constants. Components of the NQCC tensor, χij, are related to those of the electric field gradient (EFG) tensor qij by χij (α) (MHz) = (eQ α /h) × qij(α) (a. u. )

(1)

where e is the fundamental electric charge, Qα is the electric quadrupole moment of the nucleus in question (α = Br, I), and h is Planck’s constant. The coefficient eQα/h is here taken as a best-fit parameter determined by linear regression analysis of calculated qij(α) on the experimental structures of a number of molecules versus the corresponding experimental χij(α). The premise that underlies this procedure is that errors inherent in the modelincluding, but not limited to, neglect of zero-point vibrations and relativistic effects, which can be significant11

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COMPUTATIONAL DETAILS Quantum chemical calculations were undertaken to predict spectroscopic constants of sufficient accuracy for each Br 11745



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Table 3. Centrifugal Distortion Constants, Watson’s S Reduction, and Ir Representation

are systematic and may, at least in part, be corrected by the best-fit value of eQα/h. For α = Br, within the framework of this procedure, the recommended model for calculation of qij(Br) is B1LYP/ TZV(3df,3p).30,31 B1LYP is Becke’s one-parameter method with Lee−Yang−Parr correlation as implemented by Adamo and Barone.32,33 TZV are Ahlrichs bases34,35 augmented here with a set of three d- and one f- polarization functions on heavy atoms, and three p functions on hydrogens. These polarization functions are those recommended for use with Pople 6-311G bases, and were obtained online from the EMSL Basis Set Library. Then, for conversion of qij to χij, eQ79Br/h = 77.628(43) MHz/a.u. and eQ81Br/h = 64.853(40) MHz/a.u. Here, for iodine, as a TZV Ahlrichs basis is not available, a TZVPPall Ahlrichs basis obtained from the TURBOMOLE Basis Set Library II36 was used as is. For α = I, calibration of the B1LYP/6-311G(df,p) computational model for calculation of the qij(I) gives, for conversion to χij(I), eQI/h = −173.815(271) (MHz/a.u.)31 The 6-311G(df,p) are Pople-type bases, which are included in the Gaussian 03 package. For iodine, however, which is not in this package, a 6311G(d) basis37 was obtained from the EMSL library, and augmented with a f-polarization function with exponent 0.40. The results of these calculations and the equilibrium rotational constants inferred from the molecular structure are cast in Table 2.

CH279BrI

parameter Ae Be Ce Bromine χaa χbb χcc |χab| Iodine χaa χbb χcc |χab| a

a

MP2/ SDBVTZ

CCSD(T)/(aug)cc-pVTZ-PP

MP2/ SDBVTZ

24 419.3 887.9 861.5

24 612.6 888.9 862.7

24 393.2 875.0 849.3

24 586.3 875.9 850.4

315.6 −6.2 −309.4 415.7

316.5 −7.5 −309.0 413.9

264.2 −5.7 −258.5 347.1

265.0 −6.8 −258.1 345.6

−1304.8 262.1 1042.7 1284.6

−1310.6 269.5 1041.1 1278.8

−1302.8 260.1 1042.7 1285.9

−1308.6 267.5 1041.1 1280.0

B3LYP/SDBVTZ

MP2/SDBVTZ

B3LYP/SDBVTZ

MP2/SDBVTZ

DJ DJK DK d1 d2 HJ HJK HKJ HK h1 h2 h3

0.122 −8.64 406.0 −0.00613 −0.0000732 0.0301 0.353 −534 22 700 0.00407 0.000122 0.00000996

0.123 −8.61 374.0 −0.00643 −0.0000753 0.0283 0.713 −498 19 700 0.00398 0.000126 0.0000105

0.119 −8.53 405.0 −0.00589 −0.0000694 0.0290 0.347 −525 22 600 0.00385 0.000114 0.00000925

0.120 −8.49 374.0 −0.00617 −0.0000713 0.0272 0.695 −490 19 600 0.00378 0.000118 0.00000975

Quartic and sextic centrifugal distortion constants in kHz and mHz, respectively.

patterns well observable in the mmw region (where high-J transitions occur) because of the large quadrupole coupling constants similar in size as the B and C rotational constants (Table 2). During our spectroscopic investigation, few a-type lines have been recorded: 6 and 9 for the 79Br species in the mw and mmw frequency regions, respectively. The former are characterized by ΔJ = +2 and arise most probably from the mixing of wave functions due to large interaction parameters (those of the iodine hyperfine coupling constants, essentially). For the 81Br isotopologue 21 mmw a-type R-branch transitions have been measured. The dense b-type spectrum conversely exhibits strong, characteristic P1−1-, Q1−1- and R±1 1- branch transitions, where the usual ΔJΔKa ΔKc notation has been adopted. In the mw region, 420 and 434 b-type transitions have been recorded for the 79Br and 81Br isotopologues, respectively. These numbers amount to 508 and 475 in the mmw region, respectively. Figure 1 illustrates selected lines of the R0(0) fundamental transition of CH281BrI. The quadrupolar structure of the J = 119 ← 118, Ka = 1 ← 0 transition (b-type) of CH281BrI is plotted in Figure 2. Even though high-level calculations guided the experimental work, identification of the spectra of both species was difficult,

CH281Br127I

CCSD(T)/(aug)cc-pVTZ-PP

parametera

a

Table 2. Rotational Constants and Nuclear Quadrupole Coupling Constants Calculated on CCSD(T)/(aug)-ccpVTZ-PP and MP2/SDB-VTZ Optimized Molecular Structures CH279Br127I

CH281BrI

Rotational and nuclear quadrupole coupling constants in MHz.

Centrifugal Distortion Constants. Quartic and sextic CDC for each Br species were calculated at both B3LYP/SDBVTZ and MP2/SDB-VTZ levels of theory. B3LYP is Becke’s three-parameter method38 with Lee−Yang−Parr correlation.39 These results are listed in Table 3.

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ANALYSIS CH2BrI (X1A′) is a near-prolate asymmetric rotor (κ = −0.998) belonging to the Cs point group. The carbon and halogen atoms lie in the mirror plane. The two halogen nuclei have nuclear spin IBr = 3/2 and II = 5/2 and produce hyperfine

Figure 1. Nuclear electric quadrupole structure owing to 81Br observed in the JKa Kc = 11 1−00 0, F1 = 1.5−2.5 transition (F1 = J + II and F = F1 + IBr) obtained by accumulating 400 FID signals with a repetition rate of 5 Hz. Doppler components split each transition into a doublet. 11746



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were adjusted assuming a second derivative Voigt line shape with a common line width for each components. The analysis of the observed line frequencies was accomplished using Pickett’s program SPFIT.42 The molecular constants for both isotopologues in their ground vibrational state were obtained from the least-squares analyzes of a total of 943 and 930 transition frequencies respectively for CH279BrI and CH281BrI. The list of the obs. and obs. − calc. transitions frequencies can be retrieved from the Supporting Information (SI). The data were weighted according to the inverse square of the accuracy of the frequency measurements, which is 3 and 20−50 kHz in the mw and mmw regions, respectively. Completely blended lines in the mmw region were fitted as the equally weighted average of their components. The recorded spectra involve J″ ≤ 130, Ka″ ≤ 13 for CH279BrI and J″ ≤ 132, Ka″ ≤ 14 for CH281BrI. These high values were needed to ensure a reliable determination of the sextic CDC. The rms deviation of the fit is 19 kHz for CH279BrI and 20 kHz for CH281BrI.

Figure 2. Hyperfine pattern owing to 81Br and 127I nuclear spins seen in the JKa Kc = 1191 119 ← 1180 118 transition (blue, second derivative recorded). Labels identify lines with multiple (F1″, F″) pairs as lower state quantum numbers: (a) (117.5, 116), (117.5, 119), (118.5, 117), (118.5, 120); (b) (116.5, 115), (116.5, 118), (117.5, 117), (117.5, 118), (118.5, 118), (118.5, 119), (119.5, 118), (119.5, 121); (c) (119.5, 119), (119.5, 120), (116.5, 116), (116.5, 117); (d) (115.5, 114), (115.5, 117), (120.5, 119), (120.5, 122); and (e) (115.5, 115), (115.5, 116), (120.5, 120), (120.5, 121). The simulated envelope appears in dashed line (orange) and the residuals derived from the line profile deconvolution is shown in the lower trace (green).

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RESULTS AND DISCUSSION This study yielded an accurate determination of 32 molecular constants for each Br species of bromoiodomethane. Our experimental rotational and CDC parameters are provided in Table 4. This study did not allow us to determine the sextic h3

and their definite, conclusive assignment could be achieved only after high-J and high-K lines were identified in the mmw region (above 200 GHz). On one hand many unidentified lines were observed with similar intensity and/or expected hyperfine patterns in both mw and mmw investigated regions. Part of these lines may belong to vibrational excited state(s) of CH2BrI. On the other hand, spectra of both Br species are heavily blended. The analysis has been achieved with the following Hamiltonian:

Table 4. Experimental Rotational and Centrifugal Distortion Constants, Watson’s S reduction, and Ir Representationa

H = HRot + HeQq(Br) + HeQq(I) + HNSR(Br) + HNSR(I) (2)

HRot represents the Watson’s S reduction of the rotational Hamiltonian in the Ir representation and includes the CDC up to sextic order. The terms HeQq(α) with α = {Br, I} designate the quadrupole coupling interactions of nucleus α. HeQq(α) is given by Bragg40 and Casimir41 as 1 HeQq(α) = − Q α: ∇Eα (3) 6

CH279BrI

CH281BrI

A0 B0 C0 DJ DJK DK d1 d2 HJ HJK HKJ HK h1 h2 h3

24 054.89150(12) 892.391897(33) 865.008510(33) 0.1336499(37) −8.82364(11) 374.086(12) −0.00712748(56) −0.00009268(25) 0.03085(13) 1.030(12) −544.9(16) 20 073(201) 0.004574(33) 0.000147(26) 0.00001155c

24 028.96981(13) 879.362383(63) 852.728268(57) 0.1300162(55) −8.70150(10) 373.287(12) −0.00684301(47) −0.00008860(29) 0.03058(16) 1.044(9) −539.9(12) 19 943(169) 0.004360(22) 0.000214(26) 0.00001073c

a

Figures in parentheses denote one standard deviation in unit of the last quoted digit. bRotational constants in MHz. Quartic and sextic centrifugal distortion constants in kHz and mHz, respectively. cFixed empirically at the calculated value obtained by scaling the MP2/SDB -VTZ value.

which is the inner product (:) between the NQCC tensor Qα and the EFG tensor (∇Eα) due to the electrons at nucleus α. The relation between components of the NQCC tensor of nucleus α and those of the EFG tensor is given by eq 1. Finally, HNSR is written as HNSR (α) = Iα·Cα·J

parameterb

CDC, and it was therefore constrained at the computed value empirically scaled by the ratio obtained from other sextic CDC, namely 1.1. Both sets of experimental quartic and sextic CDC agree with the calculated values (Table 3). The hyperfine coupling constants used in the fitting procedure are given in Table 5, which contains the main terms (diagonal and off-diagonal) related to the NQCC tensors and a number of their J- and/or K- dependencies. The corresponding diagonal and off-diagonal χ(α) matrix elements of the NQCC tensors for the two halogens appear in Table 6 and can be directly compared with the computed values (Table 2). The consistency of the values obtained for both Br

(4)

where Cα is the nuclear spin-rotation coupling tensor of nucleus α. Each HNSR coupling term only causes small frequency shifts of the hyperfine components. The coupling scheme of angular momenta employed was F1 = J + II and F = F1 + IBr for the total angular momentum. Millimeter-wave line frequencies for individual hyperfine components were determined by a nonlinear line profile analysis using IGOR Pro (wavemetrics). This procedure is essential when, e.g., hyperfine-structure components partially overlap. The line frequencies together with the peak intensities 11747



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Table 5. Fitted Hyperfine Coupling Constants of CH2BrIa parameterb,c Bromine 3/2 χaa 3/2 χKaa × 103 1/4 χ− 1/4 χJ− × 103 |χab| Caa Cbb Ccc Iodine 3/2 χaa 3/2 χJaa × 103 3/2 χKaa × 103 1/4 χ− 1/4 χJ− × 103 |χab| |χJab| × 103 Caa Cbb Ccc

CH279BrI

CH281BrI

456.5705(17) −26.4(14) 78.271 45(55) −56.9(14) 418.0130(52) 13.62(35) 1.277(50) 2.142(52)

382.2389(18) −22.1(12) 65.248 53(53) −42.9(11) 348.9990(97) 13.84(35) 1.497(54) 2.160(53)

−1912.9705(20) −1.383(64) 57.9(16) −200.635 89(52) 0.1627(17) 1296.1970(44) 0.335(37) 17.31(16) 2.423(32) 3.503(35)

−1909.8741(19) −1.360(58) 59.9(14) −201.151 00(48) 0.1545(15) 1297.3694(69) 0.326(27) 17.07(16) 2.358(33) 3.474(34)

For best interpretation of the NQCC tensors of the two halogens, Table 6 comprises values of their matrix elements in their principal axes obtained by diagonalization in the principal inertial axes system. The z axis was assumed to lie along the carbon−halogen bond. The angle θza between this bond and the principal a axis is also provided along with the asymmetry parameter η. Once more the values derived for θza and η are homogeneous for the two isotopologues. In particular, the small values of η shows that χ(α) tensors have nearly axial symmetry around the carbon−halogen bond. One can additionally note that the 79/81Br isotopic ratio obtained for χzz, 1.197, agrees with that of the nuclear values of the quadrupole moment of the Br atoms (1.195, ref 43). The rotational constants of both isotopologues have been used in a least-squares analysis to determine the r0 structure of the molecule. To eliminate the most important part of the residual centrifugal distortion effects in the rotational constants, they were first transformed to the so-called Watson’s determinable parameters44 A0, B0, and C0, which are given in Table 7. Internal coordinates (bond lengths and angles plus one dihedral) were then directly fitted to moments of inertia in a nonlinear least-squares procedure.

a

Figures in parentheses denote one standard deviation in unit of the last quoted digit. bThe parameter χ− denotes (χbb − χcc). cConstants in MHz except NSR interaction parameters (Cii) in kHz.

Table 7. Watson’s Determinable Constants (MHz) for the Two Available Isotopomers of CH2BrIa

Table 6. Nuclear Quadrupole Coupling Constants (MHz) of CH2BrI in Principal Axes of the Inertial (a, b, c) and NQCC (x, y, z) Tensorsa Parameter Bromine χaa χbb |χab| χcc χxx χyy χzz ηb θzac Iodine χaa χbb |χab| χcc χxx χyy χzz ηb θzac

CH279BrI

CH2 BrI

304.3803(11) 4.3527(12) 418.0130(52) −308.7331(17) −289.7495(49) −308.7331(17) 598.4825(49) 0.0317 35.13

254.8259(12) 3.0841(12) 348.9990(97) −257.9100(17) −242.0487(90) −257.9100(17) 499.9587(90) 0.0317 35.08

−1275.3137(13) 236.3851(12) 1296.1970(44) 1038.9286(18) 981.0140(39) 1038.9286(18) −2019.9426(39) 0.0287 29.88

−1273.2494(13) 234.3227(12) 1297.3694(69) 1038.9267(17) 980.9902(59) 1038.9267(17) −2019.9169(59) 0.0287 29.92

isotopologue

A0

B0

C0

CH279BrI CH281BrI

24 054.891 77(12) 24 028.970 07(13)

892.383 326(34) 879.353 927(64)

864.999 968(34) 852.719 840(58)

a

Figures in parentheses denote one standard deviation in unit of the last quoted digit.

81

This has been done with the STRFIT program of Kisiel45 which is available from the PROSPE database (http://www. ifpan.edu.pl/~kisiel/prospe.htm). In order to reduce the correlation between fitted coefficients, four independent variables were floated: the CI effective bond length for the ground vibrational state, the two bond angles ∠BrCI and ∠BrCH, plus the dihedral between the (BrCI) and (BrCH) planes. The CH and CI bond lengths were kept fixed at their CCSD(T)/(aug)-cc-pVTZ-PP values. The resulting structural parameters are supplied in Table 8. A statistically significant Table 8. Experimental (r0) Structure of CH2BrIa CHb

CBr

CIb

∠HCH

∠BrCI

∠BrCH

1.0830

1.9261

2.1313

107.66

113.53

107.27

Bond lengths in Å and bond angles in degrees. Parameters fixed to their ab initio value (CCSD(T)/(aug)-cc-pVTZ-PP) in the fit. a

b

standard deviation cannot be associated with the structural parameters calculated since the fit is based on only two isotopomers. Our ab initio value for the CI bond length and the derived CBr experimental bond length agree well with those found in other halocarbon species such as in methyl bromide and methyl iodide, for which the carbon−halogen equilibrium bond length is 1.933 and 2.133 Å, respectively (ref 46, p. 219).

a

Figures in parentheses denote one standard deviation in unit of the last quoted digit. bAsymmetry parameter, η = (χxx − χyy)/χzz. cθza (degrees) is the angle between principal a and z axes, and is obtained using the relation: tan(2θza) = 2|χab|/(χaa − χbb).

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isotopologues and their excellent agreement with the ab initio results secure the identification of the rotational spectrum of CH2BrI in its vibrational ground state and illustrates the quality of the analysis. Table 5 also includes diagonal elements of both nuclear spin-rotation NSR tensors, Cα. They do not deviate between each Br species.

CONCLUSIONS Assisted by high-level quantum chemical calculations, we have performed an analysis of the rotational spectrum of the two bromine isotopologues of bromoiodomethane in its ground vibrational state recorded by Fourier transform microwave and 11748



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millimeter-wave spectroscopy. We obtained accurate molecular constants and in particular the nuclear quadrupole coupling tensors for the halogens have been determined. An experimental (r0) structure (with two internal coordinates constraints) of the molecule has also been derived. In addition, this work could form the basis for monitoring the presence of CH2BrI in the troposphere where it is supposed to play several important roles.

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ASSOCIATED CONTENT

S Supporting Information *

Vibrational frequencies and IR intensity calculated at MP2 and CCSD(T) levels of theory. Listings of obs. and obs. − calc. transition frequencies, molecular constants, and correlation matrix (input and output files with extension PAR, LIN and FIT for use with Pickett’s SPFIT for each Br isotopologue). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.B. and D.D. acknowledge financial support from the CaPPA project (Chemical and Physical Properties of the Atmosphere) funded by the French National Research Agency (ANR) through the PIA (Programme d’Investissement dAvenir) under contract ANR-10-LABX-005. D.D. also thanks CINES for access to the HPC resources under the allocation 2014-088620 made by GENCI. The Centre de Ressources Informatiques (CRI) of the Université Lille1 also provided computing time. H. O. thanks the Society of Iodine Science, Japan, for financial support.

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REFERENCES

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