Rotational spectrum of HCBr produced by 193-nm laser photolysis of bromoform

Journal of Molecular Spectroscopy 220 (2003) 113–121 www.elsevier.com/locate/jms

Rotational spectrum of HCBr produced by 193-nm laser photolysis of bromoform Chuanxi Duan,a,b M
elynda Hassouna,a Adam Walters,a,c Marc Godon,a Pascal Dr
ean,a and Marcel Bogeya,* a

b

Laboratoire de Physique des Lasers, Atomes et Mol
ecules,1 Centre d’Etudes et de Recherches Lasers et Applications,2 Universit
e des Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq Cedex, France Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, PR China c Centre d’Etude Spatiale des Rayonnements, 9 Avenue du Colonel Roche, F-31029 Toulouse Cedex, France Received 22 January 2003; in revised form 14 March 2003

Abstract The pure rotational spectrum of bromomethylene (HCBr) was studied by kinetic microwave spectroscopy between 420 and 472 GHz. The HCBr radical was produced by 193-nm ArF laser photolysis of bromoform (CHBr3 ). More than 130 rotational transitions for both HC79 Br and HC81 Br species in the ground vibrational state were measured involving 1 6 J 6 33 and 0 6 Ka 6 5. The spectra were well described by an S-reduced Watson Hamiltonian in the I r representation including the nuclear quadrupole and spin-rotation hyperfine terms. Rotational, centrifugal distortion, 79;81 Br nuclear quadrupole and spin-rotation coupling constants were derived for both HC79 Br and HC81 Br species in the ground vibrational state. Ó 2003 Elsevier Science (USA). All rights reserved. Keywords: Millimeter wave spectroscopy; Photolysis; HCBr; Hyperfine structure

1. Introduction Carbenes play an important role in organic chemistry, atmospheric chemistry, and combustion chemistry. Since Herzberg and Johns [1] first reported the electronic absorption spectra of the simplest carbene, CH2 , a large amount of experimental and theoretical research has been focused on the spectrum and the structure of simple carbenes [2,3]. In spite of these endeavours, the spectra of a large number of carbenes have not been completely analysed. The electronic spectrum of simple carbenes was found to be very complicated due to Renner–Teller and spin–orbit couplings between their three low-lying electronic states. In addition, local perturbations caused by Coriolis and Fermi interactions *

Corresponding author. Fax: +33-3-20-33-64-63. E-mail address: [email protected] (M. Bogey). 1 Unit
e Mixte de Recherche de lÕUniversit
e et du CNRS (UMR 8523). 2 F
ed
eration de Recherche CNRS (FR 2416).

contribute to this complexity. The complete understanding of the spectra and structure of simple carbenes is undoubtedly a great challenge for both experimental and theoretical spectroscopists. The atmospheric chemistry of bromine has recently received considerable attention because of its role in stratospheric ozone destruction. Bromomethylene (HCBr) is one of the most fundamental simple carbenes, and is an important photodissociation product of bromoform [4,5]. Therefore, it may play an important role in the bromine cycle of the upper atmosphere in relation to ozone depletion process [6]. In common with all simple carbenes, the HCBr radical possesses three low-lying electronic states: two singlets X~ 1 A0 , A~1 A00 and one triplet a~3 A00 . It was not until 1994 that Xu et al. [7] reported the first vibronic spectrum of the A~1 A00 –X~ 1 A0 transition of the HCBr radical. Since then, high resolution spectroscopic studies on this transition of HCBr and DCBr by the group of Trevor J. Sears [8–13] have established accurate energy levels diagram for the lowlying vibrational levels in the A~ and X~ states. The HCBr

0022-2852/03/$ - see front matter Ó 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0022-2852(03)00105-X

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radical was obtained by 193 nm photolysis of bromoform. They were only able to determine the rotational and a limited number of centrifugal distortion constants for the ground vibrational state. Tsai et al. [14] recorded the laser-induced fluorescence spectra of HCBr and were able to locate the triplet state (~ a3 A00 ) ap1 proximately 2000 cm above the zero-point level of the X~ state. They produced HCBr by discharge in a mixture of helium and bromoform in a supersonic free jet expansion. In this paper we present the measurement and analysis of the pure rotational spectrum of the HCBr radical studied by kinetic millimeter-wave spectroscopy. More accurate rotational, centrifugal distortion constants as well as 79;81 Br nuclear quadrupole and spin-rotation coupling constants are determined for both HC79 Br and HC81 Br species. The precise determination of the molecular constants for the ground vibrational state from the pure rotational spectrum should be helpful in analyzing the complicated perturbations in the electronic transition.

2. Experiment The measurements were made using a kinetic microwave spectrometer virtually identical to that described in our previous measurements of SO [15] and CBr [16]. Briefly, a conventional source-modulated millimeter and sub-millimeter wave spectrometer has been modified to allow time-resolved detection following pulsed production of reactive species by an excimer laser. Millimeter wave and UV laser beams counterpropagate in the absorption cell where reactive species are produced selectively by the photodissociation of suitable precursors. The concentration of the reactive species is modulated since after each laser pulse the species is destroyed by collision before the next laser pulse. In kinetic spectroscopy, for each frequency step, a time-resolved signal is accumulated for given numbers of laser pulses. The steady or slowly varying contribution due to stable molecules or standing waves is then easily eliminated by data processing. Only the time-varying signal in a gate set with adjustable width and delay after the laser shot is retained. The signal which would have been observed through a classical detection can also be observed simultaneously with that obtained by kinetic detection: for each frequency step it corresponds to the averaging of the entire time-resolved signal. Frequency scans using this kinetic detection permit the transitions of reactive species to be observed clearly and independently of the stable precursor spectrum. The frequency scan of the microwave source, the acquisition and treatment of the absorption signal and the synchronous trigger of the excimer laser are realized simultaneously under computer control.

HCBr radicals were produced by the 193 nm ArF laser photolysis of bromoform (CHBr3 ). The pressure was maintained at about 102 mbar by a continuous flow of bromoform. The excimer laser was fired at 50 Hz and the average output power was maintained at 80– 100 mJ. The rotational spectrum of HCBr was measured between 420 and 472 GHz with a BWO source (Thomson CSF). The source was modulated at the frequency of f ¼ 50 kHz and the output of the liquid-He cooled InSb bolometer used for detection was demodulated at 2f by a lock-in amplifier (Perkin–Elmer, 7280) with a time constant of 50 ls. The lifetime of the HCBr radical was estimated to be about 150 ls under our experimental conditions. The scanning step was 100 kHz and the temporal absorption signals at each fixed frequency were accumulated with 100 laser shots.

3. Results and analysis The initial predictions for the pure rotational spectrum of HCBr were made by the JPL program SPCAT [17] with the ground state rotational constants determined from the near-IR electronic spectrum [9] which were known at the beginning of this work. The dipole moment has been calculated ab initio [18] and the bcomponent (1.44 D) was found to be more than 5 times larger than the a-component (0.26 D). HCBr is a slightly asymmetric molecule close to a prolate rotor (j ¼ 0:998). In their paper, Marr et al. [9] were not able to determine precisely the A rotational constant and they obtained only a very coarse value with uncertainty of about 3 GHz for HC81 Br. In the region of 405–472 GHz covered by our BWO source, the b-type Q-branch transitions with Ka ¼ 1 0 were predicted to be much stronger than other types of transitions and to show a characteristic pattern. However, the uncertainty on the A value suggested a possible large shift on the positions of these transitions. Moreover, a large frequency scan was necessary to detect the typical pattern of this branch. A careful search of these transitions was made in the region of 453–472 GHz and several groups of transitions were observed. An example of one such group is shown in Fig. 1. The transitions have an easily recognizable pattern consisting of a doublet of quartets due to the 79;81 Br (I ¼ 3=2) nuclear quadrupole coupling. They were tentatively assigned to the b-type Qbranch transitions with 3 6 J 6 14 and Ka ¼ 1 0 to determine the 79;81 Br nuclear quadrupole coupling constants. The frequencies of these transitions were also sent to Chang et al. [12] and used along with the nearinfrared transitions measured by them to determine the rotational constants. With the newly determined rotational constants [12] and the 79;81 Br nuclear quadrupole coupling constants, the frequencies of the a-type R-branch transitions were predicted. The hyperfine

C. Duan et al. / Journal of Molecular Spectroscopy 220 (2003) 113–121

115

Fig. 1. The b-type Q-branch transitions 51;4  50;5 (DF ¼ 0) for HC79 Br and HC81 Br and the a-type R-branch transitions 180;18 –170;17 and 182;17 –172;16 for HC79 Br.

components of the a-type R-branch transitions were expected to be heavily blended with each other. Several strong lines around the observed b-type Q-branch transitions were tentatively assigned to the a-type R-branch transitions J ¼ 18 17, leading to the measurement and identification of a series of related transitions involving 0 6 Ka 6 5 and Kc ¼ ðJ  Ka Þ; ðJ  Ka þ 1Þ. Finally, the b-type R-branch transitions J ¼ 32 31; 33 32 and some low-J b-type Q-branch transitions ðJ ¼ 1; 2Þ were measured straightforwardly. The observed rotational transitions for HC79 Br and HC81 Br are listed in Tables 1 and 2, respectively. The typical experimental error is about 50 kHz. The spectra were well described by an S-reduced Watson Hamiltonian in the I r representation [19] including the nuclear quadrupole and spin-rotation hyperfine terms. All the observed transitions of HC79 Br and HC81 Br were fitted independently with the JPL program SPFIT [17]. The blended lines were fitted as the equally weighted average of their hyperfine components. The results of the fit are given in Tables 3 and 4. The standard deviations of the fit were 0.044 and 0.042 MHz for the HC79 Br and HC81 Br species, respectively. Even with both a- and b-type transitions included in the fit, the correlation coefficient between A and DK constants was 0.9995 for both HC79 Br and HC81 Br, respectively. The decorrelation of the parameter A from the quartic distortion constant DK would need the observation of high-J b-type P -branch transitions with Ka ¼ 2 1, which were too weak to be observed with our spectrometer. The data were not sufficient to determine all the sextic distortion constants and only HJ and HKJ were determined significantly.

4. Discussion As shown in Table 3, our determined rotational and centrifugal distortion constants in the ground vibrational state for both HC79 Br and HC81 Br species are in good agreement with those determined by Chang et al. [12] within 3r uncertainties except the values of DJK . Though the b-type transitions J1;J 1 J0;J with 2 6 J 6 14 from our initial measurement were included in the fit of Chang et al. [12], in which the average of hyperfine components frequencies was used, further measurements of the a- and b-type R-branch transitions have allowed us to determine a set of more accurate and complete constants for both the HC79 Br and HC81 Br species in the ground vibrational state. The diagonal elements vii ði ¼ a; b; cÞ of the nuclear quadrupole coupling tensors for both HC79 Br and HC81 Br species have been determined accurately in this study. The off-diagonal element vab , which is mainly related to the second-order effect of the nuclear quadrupole interaction, was not determined significantly due to the lack of observed transitions with DF 6¼ DJ . The Br-nuclear quadrupole coupling constants determined in the principal inertial axes were transformed into those in the principal quadrupole axes, assuming that the principal axis (z-axis) of the nuclear quadrupole coupling tensor is along the C–Br bond and the x-axis is perpendicular to the molecular plane. Since the angle between the C–Br bond and the a-axis is only 2.95° for both isotopic species according to the geometrical structure of HCBr [12], the off-diagonal element vab must be very small. The derived values of vii ði ¼ x; y; zÞ are given in Table 4. The uncertainty of the angle

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Table 1 Observed frequencies, calculated frequencies, and residuals (MHz) of HC79 Br in the ground vibrational statea J0

Ka0

Kc0

Fþ0

J 00

Ka00

Kc00

Fþ00

Obs.

Cal.

O)C

Av.

O ) Av.

17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 32 32 32 32 18 18 18 18 1 1 2 3 3 3 3 4 4 4 4 18 18 18 18 18 18 18 18 18

1 1 1 1 4 4 4 4 4 4 4 4 3 3 3 3 3 3 3 0 0 0 0 2 2 2 2 2 2 2 2 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5 5 5 5 5 5 5 5 4

17 17 17 17 14 13 14 13 14 13 14 13 15 14 15 14 15 14 14 17 17 17 17 16 16 16 16 15 15 15 15 16 16 16 16 32 32 32 32 18 18 18 18 0 0 1 2 2 2 2 3 3 3 3 14 13 14 13 13 14 13 14 15

19 18 16 17 19 19 16 16 18 18 17 17 19 19 18 16 17 18 17 18 19 17 16 19 18 16 17 19 16 18 17 18 19 16 17 31 34 32 33 19 20 18 17 3 2 3 2 5 3 4 3 6 4 5 20 20 17 17 19 19 18 18 20

16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 31 31 31 31 17 17 17 17 1 1 2 3 3 3 3 4 4 4 4 17 17 17 17 17 17 17 17 17

1 1 1 1 4 4 4 4 4 4 4 4 3 3 3 3 3 3 3 0 0 0 0 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 5 5 5 5 5 5 5 5 4

16 16 16 16 13 12 13 12 13 12 13 12 14 13 14 13 14 13 13 16 16 16 16 15 15 15 15 14 14 14 14 15 15 15 15 31 31 31 31 17 17 17 17 1 1 2 3 3 3 3 4 4 4 4 13 12 13 12 12 13 12 13 14

18 17 15 16 18 18 15 15 17 17 16 16 18 18 17 15 16 17 16 17 18 16 15 18 17 15 16 18 15 17 16 17 18 15 16 30 33 31 32 18 19 17 16 3 3 3 2 5 3 4 3 6 4 5 19 19 16 16 18 18 17 17 19

427386.104 427386.104 427386.104 427386.104 430148.036 430148.036 430148.036 430148.036 430149.868 430149.868 430149.868 430149.868 430330.715 430331.820 430331.820 430331.820 430331.820 430332.892 430332.892 430355.224 430355.224 430355.224 430355.224 430404.803 430404.803 430404.803 430404.803 430581.086 430581.086 430581.086 430581.086 433541.459 433541.459 433541.459 433541.459 438161.905 438165.188 438209.501 438212.022 452474.102 452474.102 452474.102 452474.102 453124.497 453132.393 453476.203 453979.281 453998.605 454014.323 454031.554 454698.786 454716.453 454742.505 454758.547 455165.381 455165.381 455166.272 455166.272 455168.277 455168.277 455168.277 455168.277 455404.313

427385.981 427385.991 427386.120 427386.139 430147.695 430147.697 430148.404 430148.406 430149.609 430149.611 430149.999 430150.001 430330.636 430331.559 430331.729 430332.111 430332.131 430332.656 430333.058 430354.739 430355.151 430355.181 430355.531 430404.447 430404.830 430404.894 430405.248 430580.501 430581.262 430580.923 430581.660 433541.118 433541.146 433541.772 433541.821 438161.895 438165.153 438209.502 438212.013 452474.012 452474.026 452474.144 452474.146 453124.440 453132.383 453476.163 453979.354 453998.611 454014.398 454031.604 454698.746 454716.454 454742.499 454758.544 455165.477 455165.477 455166.311 455166.311 455168.005 455168.005 455168.367 455168.367 455404.239

0.123 0.113 )0.016 )0.035 0.341 0.339 )0.368 )0.370 0.259 0.257 )0.131 )0.133 0.079 0.261 0.091 )0.291 )0.311 0.236 )0.166 0.485 0.073 0.043 )0.307 0.356 )0.027 )0.091 )0.445 0.585 )0.176 0.163 )0.574 0.341 0.313 )0.313 )0.362 0.010 0.035 )0.001 0.009 0.090 0.076 )0.042 )0.044 0.057 0.010 0.040 )0.073 )0.006 )0.075 )0.050 0.040 )0.001 0.006 0.003 )0.096 )0.096 )0.039 )0.039 0.272 0.272 )0.090 )0.090 0.074

427386.058 427386.058 427386.058 427386.058 430148.050 430148.050 430148.050 430148.050 430149.805 430149.805 430149.805 430149.805

0.046 0.046 0.046 0.046 )0.014 )0.014 )0.014 )0.014 0.063 0.063 0.063 0.063

430331.882 430331.882 430331.882 430331.882 430332.857 430332.857 430355.150 430355.150 430355.150 430355.150 430404.855 430404.855 430404.855 430404.855 430581.086 430581.086 430581.086 430581.086 433541.465 433541.465 433541.465 433541.465

)0.062 )0.062 )0.062 )0.062 0.035 0.035 0.074 0.074 0.074 0.074 )0.052 )0.052 )0.052 )0.052 0.000 0.000 0.000 0.000 )0.006 )0.006 )0.006 )0.006

452474.082 452474.082 452474.082 452474.082

0.020 0.020 0.020 0.020

455165.477 455165.477 455166.311 455166.311 455168.186 455168.186 455168.186 455168.186 455404.240

)0.096 )0.096 )0.039 )0.039 0.091 0.091 0.091 0.091 0.073

C. Duan et al. / Journal of Molecular Spectroscopy 220 (2003) 113–121

117

Table 1 (continued) J0

Ka0

Kc0

Fþ0

J 00

Ka00

Kc00

Fþ00

Obs.

Cal.

O)C

Av.

O ) Av.

18 18 18 18 18 18 18 18 18 18 18 5 5 5 5 18 18 18 18 18 18 18 18 6 6 6 6 7 7 7 7 18 18 18 18 8 8 8 8 9 9 9 9 10 10 10 10 11 11 11 11 12 12 12 12 33 33 33 33 13 13 13 13 14

4 4 4 4 4 4 4 0 0 0 0 1 1 1 1 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1

14 15 14 15 14 15 14 18 18 18 18 4 4 4 4 17 17 17 17 16 16 16 16 5 5 5 5 6 6 6 6 17 17 17 17 7 7 7 7 8 8 8 8 9 9 9 9 10 10 10 10 11 11 11 11 33 33 33 33 12 12 12 12 13

20 17 17 19 19 18 18 19 18 20 17 4 7 5 6 20 19 17 18 20 17 19 18 5 8 6 7 6 9 7 8 19 20 17 18 7 10 8 9 8 11 9 10 9 12 10 11 10 13 11 12 11 14 12 13 32 35 33 34 12 15 13 14 13

17 17 17 17 17 17 17 17 17 17 17 5 5 5 5 17 17 17 17 17 17 17 17 6 6 6 6 7 7 7 7 17 17 17 17 8 8 8 8 9 9 9 9 10 10 10 10 11 11 11 11 12 12 12 12 32 32 32 32 13 13 13 13 14

4 4 4 4 4 4 4 0 0 0 0 0 0 0 0 2 2 2 2 2 2 2 2 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0

13 14 13 14 13 14 13 17 17 17 17 5 5 5 5 16 16 16 16 15 15 15 15 6 6 6 6 7 7 7 7 16 16 16 16 8 8 8 8 9 9 9 9 10 10 10 10 11 11 11 11 12 12 12 12 32 32 32 32 13 13 13 13 14

19 16 16 18 18 17 17 18 17 19 16 4 7 5 6 19 18 16 17 19 16 18 17 5 8 6 7 6 9 7 8 18 19 16 17 7 10 8 9 8 11 9 10 9 12 10 11 10 13 11 12 11 14 12 13 31 34 32 33 12 15 13 14 13

455404.313 455404.806 455404.806 455406.109 455406.109 455406.109 455406.109 455599.445 455599.445 455599.445 455599.445 455602.110 455617.645 455650.252 455664.414 455671.005 455671.005 455671.005 455671.005 455879.796 455879.796 455880.491 455880.491 456688.501 456702.340 456739.273 456751.741 457958.554 457970.870 458010.863 458022.104 458988.771 458988.771 458988.771 458988.771 459413.095 459424.145 459466.595 459476.548 461053.105 461063.347 461107.487 461116.538 462880.246 462889.592 462935.24 462943.649 464895.838 464904.592 464951.440 464959.195 467101.534 467109.735 467157.660 467164.875 468069.343 468072.608 468116.362 468118.784 469499.244 469506.979 469555.804 469562.569 472090.758

455404.242 455404.881 455404.884 455405.861 455405.863 455406.226 455406.228 455599.046 455599.438 455599.479 455599.810 455602.097 455617.715 455650.231 455664.424 455670.715 455671.021 455671.109 455671.396 455879.659 455880.027 455880.365 455880.719 456688.533 456702.343 456739.213 456751.752 457958.529 457970.871 458010.851 458022.028 458988.506 458988.558 458989.102 458989.129 459413.008 459424.167 459466.480 459476.545 461053.130 461063.329 461107.464 461116.618 462880.236 462889.647 462935.257 462943.656 464895.827 464904.585 464951.423 464959.190 467101.554 467109.766 467157.653 467164.886 468069.347 468072.554 468116.402 468118.838 469499.215 469506.967 469555.773 469562.550 472090.754

0.071 )0.075 )0.078 0.248 0.246 )0.117 )0.119 0.399 0.007 )0.034 )0.365 0.013 )0.070 0.021 )0.010 0.290 )0.016 )0.104 )0.391 0.137 )0.231 0.126 )0.228 )0.032 )0.003 0.060 )0.011 0.025 )0.001 0.012 0.076 0.265 0.213 )0.331 )0.358 0.087 )0.022 0.115 0.003 )0.025 0.018 0.023 )0.080 0.010 )0.055 )0.017 )0.007 0.011 0.007 0.018 0.005 )0.020 )0.031 0.007 )0.011 )0.004 0.054 )0.040 )0.054 0.029 0.012 0.031 0.019 0.004

455404.240 455404.883 455404.883 455406.045 455406.045 455406.045 455406.045 455599.443 455599.443 455599.443 455599.443

0.073 )0.077 )0.077 0.064 0.064 0.064 0.064 0.002 0.002 0.002 0.002

455671.060 455671.060 455671.060 455671.060 455879.843 455879.843 455880.542 455880.542

)0.055 )0.055 )0.055 )0.055 )0.047 )0.047 )0.051 )0.051

458988.824 458988.824 458988.824 458988.824

)0.053 )0.053 )0.053 )0.053

118

C. Duan et al. / Journal of Molecular Spectroscopy 220 (2003) 113–121

Table 1 (continued) J0

Ka0

Kc0

Fþ0

J 00

Ka00

Kc00

Fþ00

Obs.

Cal.

O)C

14 14 14

1 1 1

13 13 13

16 14 15

14 14 14

0 0 0

14 14 14

16 14 15

472098.089 472147.687 472154.143

472098.115 472147.741 472154.125

)0.026 )0.054 0.018

Av.

O ) Av.

a Fþ ¼ F þ 1=2. The O ) Av. heading refers to the residuals of observed positions of blended lines and calculated average positions of hyperfine components.

Table 2 Observed frequencies, calculated frequencies, and residuals (MHz) of HC81 Br in the ground vibrational statea J0

Ka0

Kc0

Fþ0

J 00

Ka00

Kc00

Fþ00

Obs.

Cal.

O)C

Av.

O ) Av.

17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 32 32 32 32 18 18 18 18 1 1 2 2 18 18 18 18

1 1 1 1 4 4 4 4 4 4 4 4 3 3 3 3 3 3 3 3 0 0 0 0 2 2 2 2 2 2 2 2 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 5 5 5 5

17 17 17 17 14 13 14 13 14 13 14 13 15 15 14 15 15 14 14 14 17 17 17 17 16 16 16 16 15 15 15 15 16 16 16 16 32 32 32 32 18 18 18 18 0 0 1 1 14 13 14 13

19 18 16 17 19 19 16 16 18 18 17 17 19 16 19 18 17 16 18 17 18 19 17 16 19 18 16 17 19 16 18 17 18 19 16 17 31 34 32 33 19 20 17 18 3 2 4 3 20 20 17 17

16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 31 31 31 31 17 17 17 17 1 1 2 2 17 17 17 17

1 1 1 1 4 4 4 4 4 4 4 4 3 3 3 3 3 3 3 3 0 0 0 0 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 5 5 5 5

16 16 16 16 13 12 13 12 13 12 13 12 14 14 13 14 14 13 13 13 16 16 16 16 15 15 15 15 14 14 14 14 15 15 15 15 31 31 31 31 17 17 17 17 1 1 2 2 13 12 13 12

18 17 15 16 18 18 15 15 17 17 16 16 18 15 18 17 16 15 17 16 17 18 16 15 18 17 15 16 18 15 17 16 17 18 15 16 30 33 31 32 18 19 16 17 3 3 4 3 19 19 16 16

425930.223 425930.223 425930.223 425930.223 428672.243 428672.243 428673.007 428673.007 428674.105 428674.105 428674.105 428674.105 428854.076 428855.101 428855.101 428855.101 428855.101 428855.101 428856.146 428856.146 428879.949 428879.949 428879.949 428879.949 428928.141 428928.141 428928.141 428928.141 429101.865 429101.865 429101.865 429101.865 432043.414 432043.414 432043.414 432043.414 434847.123 434850.113 434887.004 434889.132 450932.905 450932.905 450932.905 450932.905 453161.297 453166.789 453502.957 453511.123 453605.009 453605.009 453605.781 453605.781

425930.109 425930.118 425930.224 425930.242 428672.258 428672.260 428672.960 428672.962 428673.903 428673.905 428674.265 428674.267 428854.036 428854.549 428854.940 428854.971 428855.325 428855.452 428855.878 428856.232 428879.573 428879.922 428879.936 428880.218 428927.812 428928.138 428928.197 428928.492 429101.430 429101.792 429102.069 429102.406 432043.207 432043.238 432043.735 432043.786 434847.075 434850.062 434886.997 434889.185 450932.858 450932.870 450932.968 450932.968 453161.253 453166.781 453502.964 453511.150 453604.948 453604.948 453605.808 453605.808

0.114 0.105 )0.001 )0.019 )0.015 )0.017 0.047 0.045 0.202 0.200 )0.160 )0.162 0.040 0.552 0.161 0.130 )0.224 )0.351 0.268 )0.086 0.376 0.027 0.013 )0.269 0.329 0.003 )0.056 )0.351 0.435 0.073 )0.204 )0.541 0.207 0.176 )0.321 )0.372 0.048 0.051 0.007 )0.053 0.047 0.035 )0.063 )0.063 0.044 0.008 )0.007 )0.027 0.061 0.061 )0.027 )0.027

425930.173 425930.173 425930.173 425930.173 428672.259 428672.259 428672.961 428672.961 428674.085 428674.085 428674.085 428674.085

0.050 0.050 0.050 0.050 )0.016 )0.016 0.046 0.046 0.020 0.020 0.020 0.020

428855.047 428855.047 428855.047 428855.047 428855.047 428856.055 428856.055 428879.912 428879.912 428879.912 428879.912 428928.160 428928.160 428928.160 428928.160 429101.924 429101.924 429101.924 429101.924 432043.492 432043.492 432043.492 432043.492

0.054 0.054 0.054 0.054 0.054 0.091 0.091 0.037 0.037 0.037 0.037 )0.019 )0.019 )0.019 )0.019 )0.059 )0.059 )0.059 )0.059 )0.078 )0.078 )0.078 )0.078

450932.916 450932.916 450932.916 450932.916

)0.011 )0.011 )0.011 )0.011

453604.948 453604.948 453605.808 453605.808

0.061 0.061 )0.027 )0.027

C. Duan et al. / Journal of Molecular Spectroscopy 220 (2003) 113–121

119

Table 2 (continued) J0

Ka0

Kc0

Fþ0

J 00

Ka00

Kc00

Fþ00

Obs.

Cal.

O)C

Av.

O ) Av.

18 18 18 18 18 18 18 18 18 18 18 18 3 3 18 18 18 18 18 18 18 18 18 18 18 18 3 3 18 18 18 18 4 4 4 4 5 5 5 5 6 6 6 6 18 18 18 18 7 7 7 7 8 8 8 8 9 9 9 9 10 10 10 10

5 5 5 5 4 4 4 4 4 4 4 4 1 1 3 3 3 3 3 3 0 3 0 0 3 0 1 1 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

13 14 13 14 15 14 15 14 15 14 15 14 2 2 16 16 16 16 15 15 18 15 18 18 15 18 2 2 16 16 16 16 3 3 3 3 4 4 4 4 5 5 5 5 17 17 17 17 6 6 6 6 7 7 7 7 8 8 8 8 9 9 9 9

19 19 18 18 20 20 17 17 19 19 18 18 2 5 20 17 19 18 20 17 19 19 18 20 18 17 3 4 20 17 19 18 3 6 4 5 4 7 5 6 5 8 6 7 19 20 17 18 6 9 7 8 7 10 8 9 8 11 9 10 9 12 10 11

17 17 17 17 17 17 17 17 17 17 17 17 3 3 17 17 17 17 17 17 17 17 17 17 17 17 3 3 17 17 17 17 4 4 4 4 5 5 5 5 6 6 6 6 17 17 17 17 7 7 7 7 8 8 8 8 9 9 9 9 10 10 10 10

5 5 5 5 4 4 4 4 4 4 4 4 0 0 3 3 3 3 3 3 0 3 0 0 3 0 0 0 2 2 2 2 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

12 13 12 13 14 13 14 13 14 13 14 13 3 3 15 15 15 15 14 14 17 14 17 17 14 17 3 3 15 15 15 15 4 4 4 4 5 5 5 5 6 6 6 6 16 16 16 16 7 7 7 7 8 8 8 8 9 9 9 9 10 10 10 10

18 18 17 17 19 19 16 16 18 18 17 17 2 5 19 16 18 17 19 16 18 18 17 19 17 16 3 4 19 16 18 17 3 6 4 5 4 7 5 6 5 8 6 7 18 19 16 17 6 9 7 8 7 10 8 9 8 11 9 10 9 12 10 11

453607.287 453607.287 453607.287 453607.287 453842.186 453842.186 453842.809 453842.809 453843.845 453843.845 453843.845 453843.845 454016.596 454033.408 454035.868 454036.827 454036.827 454036.827 454036.827 454036.827 454038.049 454038.049 454038.049 454038.049 454038.049 454038.049 454046.011 454060.645 454314.089 454314.089 454314.089 454314.089 454731.541 454746.992 454768.351 454781.898 455628.892 455642.393 455669.281 455681.299 456707.890 456719.843 456750.303 456761.002 457403.045 457403.045 457403.045 457403.045 457969.099 457979.817 458012.938 458022.295 459413.386 459423.177 459458.334 459466.776 461042.193 461051.157 461087.790 461095.501 462856.601 462864.796 462902.713 462909.843

453607.127 453607.127 453607.483 453607.483 453842.258 453842.260 453842.890 453842.893 453843.652 453843.655 453843.989 453843.992 454016.615 454033.430 454035.837 454036.292 454036.632 454036.953 454037.041 454037.495 454037.705 454037.840 454038.027 454038.072 454038.161 454038.328 454046.044 454060.637 454313.609 454313.921 454314.199 454314.498 454731.598 454746.981 454768.295 454781.884 455628.884 455642.436 455669.232 455681.250 456707.869 456719.855 456750.337 456760.958 457402.864 457402.915 457403.343 457403.376 457969.095 457979.818 458012.930 458022.401 459413.496 459423.205 459458.287 459466.821 461042.224 461051.113 461087.732 461095.500 462856.606 462864.825 462902.685 462909.819

0.160 0.160 )0.196 )0.196 )0.072 )0.074 )0.081 )0.084 0.193 0.190 )0.144 )0.147 )0.019 )0.022 0.031 0.535 0.195 )0.126 )0.214 )0.668 0.344 0.209 0.022 )0.023 )0.112 )0.279 )0.033 0.008 0.480 0.168 )0.110 )0.409 )0.057 0.011 0.056 0.014 0.008 )0.043 0.049 0.049 0.021 )0.012 )0.034 0.044 0.181 0.130 )0.298 )0.331 0.004 )0.001 0.008 )0.106 )0.110 )0.028 0.047 )0.045 )0.031 0.044 0.058 0.001 )0.005 )0.029 0.028 0.024

453607.305 453607.305 453607.305 453607.305 453842.259 453842.259 453842.891 453842.891 453843.822 453843.822 453843.822 453843.822

)0.018 )0.018 )0.018 )0.018 )0.073 )0.073 )0.082 )0.082 0.023 0.023 0.023 0.023

454036.883 454036.883 454036.883 454036.883 454036.883 454038.022 454038.022 454038.022 454038.022 454038.022 454038.022

)0.056 )0.056 )0.056 )0.056 )0.056 0.027 0.027 0.027 0.027 0.027 0.027

454314.057 454314.057 454314.057 454314.057

0.032 0.032 0.032 0.032

457403.124 457403.124 457403.124 457403.124

)0.079 )0.079 )0.079 )0.079

120

C. Duan et al. / Journal of Molecular Spectroscopy 220 (2003) 113–121

Table 2 (continued) J0

Ka0

Kc0

Fþ0

J 00

Ka00

Kc00

Fþ00

Obs.

Cal.

O)C

33 33 33 33 11 11 11 11 12 12 12 12 13 13 13 13 14 14 14 14

0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

33 33 33 33 10 10 10 10 11 11 11 11 12 12 12 12 13 13 13 13

32 35 33 34 10 13 11 12 11 14 12 13 12 15 13 14 13 16 14 15

32 32 32 32 11 11 11 11 12 12 12 12 13 13 13 13 14 14 14 14

1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

32 32 32 32 11 11 11 11 12 12 12 12 13 13 13 13 14 14 14 14

31 34 32 33 10 13 11 12 11 14 12 13 12 15 13 14 13 16 14 15

464643.186 464646.205 464682.605 464684.733 464858.134 464865.786 464904.592 464911.298 467048.401 467055.619 467095.361 467101.534 469429.242 469436.033 469476.607 469482.369 472002.518 472008.953 472050.248 472055.615

464643.190 464646.143 464682.656 464684.784 464858.123 464865.789 464904.681 464911.285 467048.403 467055.609 467095.381 467101.537 469429.220 469436.041 469476.581 469482.355 472002.491 472008.985 472050.210 472055.657

)0.004 0.062 )0.051 )0.051 0.011 )0.003 )0.089 0.013 )0.002 0.010 )0.020 )0.003 0.022 )0.008 0.026 0.014 0.027 )0.032 0.038 )0.042

a

Av.

O ) Av.

See Footnote to Table 1.

Table 3 Rotational and centrifugal distortion constants (MHz) of HC79 Br and HC81 Br in the ground vibrational statea Parameter

A B C DJ DJK DK 103 d1 103 d2 106 HJ 103 HKJ a b

HC79 Br

HC81 Br

This work

Chang et al.b

This work

Chang et al.b

465709.26(99) 12855.367(10) 12492.091(10) 0.019023(18) 0.73441(27) 108.7(10) )0.4935(22) )0.0349(21) )0.0267(87) 0.373(11)

465708.9(51) 12856.3(11) 12494.2(10) 0.0199(10) 0.494(39) 105.2(48)

465701.05(99) 12810.597(10) 12449.809(10) 0.018915(18) 0.73022(25) 103.8(10) )0.4887(22) )0.0356(28) )0.0171(85) 0.371(10)

465701.5(48) 12808.6(11) 12449.5(9) 0.01637(93) 0.402(39) 102.8(45)

One standard deviation in parentheses. Values are converted from the molecular constants (cm1 ) in [12].

Table 4 Nuclear quadrupole and spin-rotation coupling constants of HC79 Br and HC81 Br in the ground vibrational statea Parameter

HC79 Br

HC81 Br

vaa (MHz) vbb (MHz) vcc ¼ vxx (MHz) vyy (MHz) vzz (MHz) g Caa (MHz) Cbb (MHz) 106 Kaa 106 Kbb

348.16(83) )398.13(50) 50.03(50)b )400.12(43) 350.15(89) 1.286(3) 3.539(74) 0.0679(36) 7.60(16) 5.28(28)

291.68(84) )333.08(50) 41.40(50)b )334.74(44) 293.34(89) 1.282(4) 3.765(72) 0.0726(36) 8.08(15) 5.67(28)

a b

One standard deviation in parentheses. Derived value.

between the C–Br bond and the a axis was taken as 0.05° to calculate the uncertainties of vii ði ¼ y; zÞ. The 79=81 Br isotopic ratio of vzz is estimated as 1.194, which agrees very well with the ratio of the nuclear quadrupole constants of the Br atoms (1.197). The asymmetry parameters g defined as ðvxx  vyy Þ=vzz are 1.286(3) and 1.282(4) for HC79 Br and HC81 Br, respectively. Similar results, 1.35(9) and 1.31(11), were reported for HC35 Cl and HC37 Cl [20], respectively. The large g constants indicate that the electronic distributions about the C–Br bond are highly asymmetric. Two nuclear spin-rotation coupling constants, Caa and Cbb , have also been determined accurately in this study. The Ccc constant could not be determined significantly from our data. The 79=81 Br isotopic ratios

C. Duan et al. / Journal of Molecular Spectroscopy 220 (2003) 113–121

of the parameter Kaa and Kbb defined by Kii ¼ Cii = Bi ði ¼ a; bÞ, where Bi is the relevant rotational constant, are 0.941 and 0.931 being very close to the ratio of the magnetic dipole moments of the Br atoms (0.9277). The large Caa values for HCBr are due to the large rotational constant A and the low-lying A~1 A00 electronic state. A similar result is reported for HCF [21], where the Caa constant is as large as 4.19 MHz. The Cbb values for HCBr are larger than the values reported for HC35 Cl and HC37 Cl [20], where the Cbb constant is 0.02497(51) and 0.02023(54) MHz, respectively. The 79 Br=35 Cl and 81 Br=37 Cl Cbb ratios are 2.56 and 3.32, respectively, which are very close to the 2.72 and 3.59 ratios of the atomic nuclear magnetic moment [22], respectively.

Acknowledgments The CERLA is partly supported by the French Research Ministry, the Nord-Pas de Calais Region, and the European Funds for Regional Economic Development. C. Duan thanks the Embassy of France in China for supporting his stay in France through a ‘‘Bourse Doctorale en Alternance.’’

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