An ab initio investigation on the ground electronic state of chlorine monoxide and its singly charged cation and anion

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 453–460

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

An ab initio investigation on the ground electronic state of chlorine monoxide and its singly charged cation and anion Song Li ⇑, Shan-Jun Chen, Yan Chen School of Physics and Optoelectronic Engineering, Yangtze University, Jingzhou, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 High level computations and basis set

25

15.6 15.5

MRCI/CBS

15.4

20

15.3 0.26 0.28 0.30 0.32

15

E/eV

extrapolation methods are employed.  Molecular structures, spectroscopic constants and vibrational levels are predicted.  Capable of supporting further experimental and theoretical researches.

-

1.2

10

ClO + ClO ClO

0.8 0.4

5

0.0 0.14

0.16

0.18

0 0.1

0.2

0.3

0.4

0.5

R/nm

a r t i c l e

i n f o

Article history: Received 30 April 2015 Received in revised form 17 July 2015 Accepted 28 July 2015 Available online 29 July 2015 Keywords: ClO ClO+ ClO Equilibrium geometrical parameters Potential energy curves Spectroscopic constants Vibrational energy levels

a b s t r a c t The MRCI method has been utilized to calculate the equilibrium structure of the ground electronic state for the ClO radical and its singly charged cation and anion, ClO+ and ClO. The augmented correlation-consistent basis sets up through sextuple-zeta quality are used to derive equilibrium structural parameters, potential energy curves and spectroscopic constants of the systems. Two extrapolating schemes enable us to remove the basis set truncation error and to estimate the complete basis set limit. Corrections of core-valence correlation and relativistic effect are included in our calculations. By solving the radial Schrödinger equation of nuclear motion, the vibrational energy levels as well as rotational and centrifugal distortion constants of the ground electronic states for the three species are obtained. Ionization potentials and the electron affinities are also obtained on the RCCSD(T)/AV6Z level. The knowledge extracted from this work are anticipated to extend our understanding on molecular characteristics of the ClOn (n = 1, 0, +1) systems and can serve as guidelines for further experimental or theoretical researches. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Halogen monoxides play an important role in the atmospheric chemistry as they are responsible for the ozone depletion in the stratosphere [1,2] and for the oxidation potential of the troposphere [3–5], etc. It is essential to get deeper insights into these environmentally significant catalytic cycles and processes, and that ⇑ Corresponding author at: School of Physics and Optoelectronic Engineering, Yangtze University, Nanhuan Road 1, Jingzhou, China. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.saa.2015.07.100 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

requires a more thorough understanding of those important species. We have presented high-level theoretical investigation on the ground electronic state of fluorine monoxide, including FO+ and FO molecular ions [6]. As part of a series of researches on halogen monoxides, we report molecular properties of neutral and ionic chlorine monoxide in this paper. The existence of the ClO radical was first experimentally confirmed by Pannetier and Gaydon in 1948 [7]. Thereafter, a variety of experimental techniques have been performed to study ClO, including methods of flash photolysis [8–10], electron resonance

454

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 453–460

spectroscopy [11], microwave spectroscopy [12–15], ultraviolet spectroscopy [16,17], matrix isolation [18], laser magnetic resonance spectroscopy [19], infrared spectroscopy [20,21], multiphoton ionization spectroscopy [22,23], laser-induced fluorescence spectroscopy [24], resonance-enhanced multiphoton ionization [25–27] and velocity-map ion-imaging [28–30]. Efforts have also been made on the accompanying theoretical analyses [31–36]. Several groups have devoted to investigations of ionic ClO species. Photoelectron spectroscopy of ClO molecular ion was reported by Gilles et al. [37], and spectroscopic constants and thermochemical properties of the anion were deduced. Anion-ZEKE-photoelectron spectra of ClO was recorded by Distelrath and Boesl [38], but no parameter related with the anion was reported, except vibrational frequencies and electron affinity of its corresponding neutral molecule. The method of CISD [39], CEPA [39], MPn [33,39,40], CCSD(T) [31,32], B3LYP [41] and MRCI [34] was also been used to explore the species. All these computations were carried out with small to moderate basis sets, except Peterson et al. who utilized correlation-consistent basis set up to Quintuple-zeta quality [32]. The ab initio calculations for ClO+ were carried out with several methodologies, including MP4SDQ, CISD and CASSCF, coupled with a basis set of 101 contracted Gaussian-type orbitals by Peterson and Woods [42]. However, the molecular constants derived from each method exhibit relative large deviations. Ma et al. determined the equilibrium internuclear distance of the cation at the MP2/6-31G(d) level of theory and enthalpy and heat of formation were obtained by the G2 procedure [33]. The low-lying singlet and triplet valence states of ClO+ were calculated at the MRCI/AVQZ level by Lane and A.J. Orr-Ewing [43]. Geometrical parameters for each state were obtained. A systematical study of structural parameters and molecular constants was performed with the MRCI method by Qi et al. [34]. More favorable data were presented by extrapolating results of the AVDZ, AVTZ and AVQZ calculations to the complete basis set (CBS) limit. To the best of our knowledge, the only experimental research concerning ClO+ was carried out by Freeman et al. with the velocity-map ion-imaging technique [44]. The UV photodissociation of the vibrational states with v = 2–45 of the cation was reported. In order to model their experimental results, the group also performed theoretical calculations on the ground and several low-lying excited states. Although the potential energy curves were illustrated, no geometrical parameter or spectroscopic constant was presented. In this work, we have performed high-level computations on the electronic and geometrical structures of ClO, ClO+ and ClO. Molecular properties and spectroscopic constants for the three species are determined. The paper is organized as follows. It begins by describing the theoretical methods and basis sets used in this study, in Section 2. Subsequently, it presents the computation results together with corresponding discussions in Section 3, including the equilibrium geometrical parameters, potential energy curves (PECs), spectroscopic constants, vibrational energy levels, ionization potentials and electron affinities of the systems.

2. Computational details The calculations have been performed in the C 2v subgroup of C 1v main group with the MOLPRO package [45]. Average atomic masses are used in the computations. The augmented correlation-consistent basis sets aug-cc-pVXZ (X = Q, 5, 6) of Dunning and co-workers, which are abbreviated to AVXZ (X = Q, 5, 6) for simplicity, are utilized. The largest basis sets AV6Z are (17s, 11p, 6d, 5f, 4g, 3h, 2i) primitive Gaussian functions contracted to [8s, 7p, 6d, 5f, 4g, 3h, 2i] for O atom, and (22s, 15p, 6d, 5f, 4g, 3h, 2i) to [9s, 8p, 6d, 5f, 4g, 3h, 2i] for Cl atom.

According to the T1 diagnostics, the multi-reference character is quite significant for all three systems, especially at the internuclear separation around their equilibrium internuclear distances. In order to take into account the electron correlation effect sufficiently, our energy scans are performed at the multi-reference configuration interaction (MRCI) level [46] and based on a complete active space SCF (CASSCF) wavefunction [47]. Higher-order CI terms are corrected by Davidson correction. The PECs of all species are calculated over the internuclear separation range from 0.09 nm to 1.0 nm at intervals of 0.005 nm. In the region of the minimum on each PEC, the scanning step declines to 0.002 nm. Processes of extrapolating to the CBS limits are performed in this work. Although extrapolating the reference energy and the electron correlation energy separately is more meaningful, we have used two schemes to extrapolate the total energy because (1) it is more convenient to deal with procedures of extrapolating the total energy than to treat reference energy and electron correlation energy respectively, and (2), which is more significant, compared with the basis set truncation error of the correlation energy, contribution from that of the reference energy is negligible with basis set quality higher than quadruple-zeta [32]. The total energies are extrapolated with the mixed exponential/Gaussian function of Peterson et al. [48]:

EðXÞ ¼ ECBS þ b exp½ðX  1Þ þ c exp½ðX  1Þ2 

ð1Þ

where X = 4, 5, 6 for AVQZ to AV6Z respectively, and a formula that involves the reciprocal of lmax [49]: 3

EðXÞ ¼ ECBS þ a=lmax

ð2Þ

where lmax is the highest angular momentum present in the basis set. The average of these two extrapolation results is our estimation of the CBS limit, and is denoted by CBS throughout this paper. The corrections of the core-valence (CV) correlation are carried out with the CV basis set aug-cc-pCV5Z, and the results are denoted as +CV in this paper. The relativistic correction is taken into account with the aug-cc-pV5Z-DK basis set coupled with the third-order Douglas-Kroll Hamiltonian (DKH3) approximation [50,51], which is denoted as +DK. As the basis set superposition error (BSSE) is anticipated to have a minor impact on the present systems, it has been ignored in our computations. The Murrell–Sorbie (M–S) potential energy function [52], which has been used to deduce the PECs of several species by our group [6,53–56], is used in the present fitting processes. The M–S potential energy function is given by the following expression,

VðqÞ ¼ De 1 þ

! n X ai qi expða1 qÞ

ð3Þ

i¼1

where q ¼ R  Re . R and Re are internuclear distance and equilibrium internuclear distance respectively. De is the well depth. In this work, all parameters are floated in our fitting by using the M–S function with n = 12. Based on our computational scheme, uncertainties are expected to be less than 0.0002 nm for Re and 0.01 eV for the dissociation energy D0. According to our equilibrium geometrical parameters and PECs, vibrational energy levels of the ground electronic states for different isotopes of the three species are obtained by numerically solving the one-dimensional Schrödinger equation of nuclear motion with the LEVEL 8.0 program package [57]. The rotational and centrifugal distortion constants for each vibrational energy level have also been determined.

455

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 453–460

3. Results and discussions 3.1. PECs and equilibrium internuclear distances According to our energy scans, PECs of the three systems are deduced by fitting the extrapolated energy points with the 12-parameter M–S function. The PECs are illustrated in Fig. 1.

25

15.6 15.5

MRCI/CBS

15.4

20

15.3 0.26 0.28 0.30 0.32

E/eV

15

-

1.2

10

ClO + ClO ClO

0.8 0.4

5

0.0 0.14

0.16

0.18

0 0.1

0.2

0.3

0.4

0.5

R/nm Fig. 1. PECs of the ground states of ClO, ClO+ and ClO.

One can see that potential well on the PEC of ClO, denoted by the black line, is the shallowest one, which corresponds to the least dissociation energy. The curve with a relative flat characteristic in the internuclear distance range from 0.20 nm to 0.35 nm is attributed to ClO(red line), and the third PEC (blue line) represented with the greatest absolute energy at Re and the largest dissociation energy belonged to ClO+. The electronic configuration of the ground electronic state of ClO radical is 1r2 2r2 3r2 4r2 1p4 5r2 6r2 2p4 7r2 3p3 . By adding an extra electron into the antibonding p molecular orbital, the anion is generated and its bond length is expected to be expanded compared to that of the neutral ClO. On the contrary, the cation is formed with removal of an electron from the antibonding p orbital of ClO and poses a shorter internuclear distance between Cl and O atoms as well as a stronger bond than the neutral. The equilibrium geometrical parameters of the systems obtained in this work are listed in Table 1, and the fitting parameters as well as the force constants that derived are collected in Table 2. Our final estimations of the equilibrium internuclear distances are 0.15657 nm, 0.14739 nm and 0.16911 nm for ClO, ClO+ and ClO respectively, which are denoted as CBS + CV + DK. These values reproduce our expectation of Re decreases in the order of  þ ClO > ClO > ClO favorably. Generally, equilibrium internuclear distances Re decrease with improving basis set quality employed in our MRCI computations. The effect of enlarging the basis set from AVQZ to AV5Z is found to decrease Re of 0.001 nm, while the decrement is 0.0002 nm with increasing basis set size from AV5Z to AV6Z. Extrapolating to the CBS limit further reduces Re of 0.0002 nm.

Table 1 Equilibrium geometrical parameters of ClO (X 2 P), ClO+ (X 3 R ) and ClO (X 1 Rþ ). Re/nm

E/a.u.

D0/eV

AVQZ AV5Z AV6Z CBS CBS + CV CBS + DK CBS + CV + DK Exp [11] Exp [13] Exp [14] Exp [17]a Exp [15]b Exp. [37] Theory [33]c Theory [34]d Theory [32]e Theory [35]f Theory [31]g Theory [39]h Theory [40]i Theory [41]j Theory [42]k Theory [43]l a

0.15806 0.15716 0.15692 0.15662 0.15647 0.15671 0.15657 0.1569(4) 0.1569(1) 0.156959(1) 0.156965(3) 0.15695394 – 0.1607 0.15629 0.15683 0.1576 0.1576 – – – – –

Re/nm

E/a.u.

D0/eV

ClO+

ClO 534.78693 534.80121 534.80670 534.81261 534.94089 536.27861 536.40689 – – – – – – – – – 535.38538 – – – – – –

2.581 2.685 2.662 2.674 – – – – – – 2.7504(4) – – 2.625 2.62439 2.801 – – – – – – –

0.14873 0.14781 0.14765 0.14748 0.14733 0.14753 0.14739 – – – – – – 0.1468 0.148244 – – – – – – 0.14730 0.14837

Re/nm

E/a.u.

D0/eV

534.87236 534.88703 534.89277 534.89888 535.02650 536.36446 536.49209 – – – – – – – – – – 534.965123 – 534.690279 – – –

3.458 3.540 3.514 3.529 – – – – – – – – – – 3.03555 1.4605 – 3.65 – – 1.40 – –

ClO 534.38904 534.40304 534.40824 534.41398 534.54254 535.88045 536.00900 – – – – – – – – – – – – – – 534.33952 534.39275822

4.612 4.856 4.841 4.867 – – – – – – – – – – 5.30707 – – – – – – 5.247 4.834(5)

0.17035 0.16954 0.16936 0.16914 0.16889 0.16936 0.16911 – – – – – 0.1673(8) 0.1716 0.169719 0.16798 – 0.1688 0.16780 0.1741 0.1685 – –

Result of the 35ClO isotope. Effective constant. c MP2/6-31G(d) calculation. d MRCI/CBS(DTQ) calculation. e CCSD(T)/CBS calculation including contributions from several corrections for 35ClO isotope. D0 of ClO was the energy relative to the Cl(1S0) + O(3P2) dissociation channel. f B3LYP/cc-pV5Z calculation. g CCSD(T)/AVQZ calculation. D0 was derived on the CCSDT/CBS level. h MP4SDQ/modified Gaussian-type orbitals calculation. i MP4(SDTQ)/electric-field optimized basis sets calculation. j B3LYP/CBS(DTQ5) calculation. D0 was the energy relative to the Cl(1S0) + O(3P2) dissociation channel. k CISD/modified Gaussian-type orbitals calculation. l MRCI/AVQZ calculation. b

456

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 453–460

Table 2 Fitting parameters of the M–S function and force constants of ClO, ClO+ and ClO in their ground electronic states. Parameters

a1/nm1 a2/nm2 a3/nm3 a4/nm4 a5/nm5 a6/nm6 a7/nm7 a8/nm8 a9/nm9 a10/nm10 a11/nm11 a12/nm12 f2/aJnm2 f3/aJnm3 f4/aJnm4 ERMS/cm1

ClO+

ClO

ClO

AV6Z

CBS

AV6Z

CBS

AV6Z

CBS

27.60590 159.5409 336.482 24335.22 188120.4 1362795.1 8842653.3 76136647.7 772,830,569 2545924416 3731372004 2070548046 469.98 28899.9 1,548,583 4.7

28.15856 141.9192 475.631 25758.20 191419.1 1638877.2 10600679.2 82501478.0 868,982,361 2907091508 4315348459 2422740429 470.13 28720.3 1,542,685 4.2

45.56699 547.2688 4026.259 47815.44 842644.1 12695908.6 292130396.5 2035935030.0 21448288749 234069571431 739408379594 777139643552 772.06 50137.9 2,007,905 7.7

45.31845 543.1429 3275.332 31232.06 1067094.4 8721956.1 351611357.6 1875974796.9 27804924128 273958289884 841664806100 872133387488 764.49 45919.6 1,644,223 9.2

32.46832 304.8296 2535.655 680.58 33578.3 50026.6 1043289.7 3114057.6 210,335,122 1064601126 1920823814 955,625,022 253.38 13842.9 819,277 3.1

30.65287 241.3549 2418.745 1100.01 115598.9 571197.8 9998160.2 3482576.9 356,481,343 1634425216 3035245021 2162168333 261.69 15880.2 962,354 6.4

The influence of the CV effect on the equilibrium bond length is quite significant. To account for this correction, the core basis set should be large enough to give properly the correlation effect. With basis set superior to quadruple-zeta quality, it is found that just minor variation is introduced by the CV correction to the molecular properties. In the present work, we have used the quintuple-zeta basis set (aug-cc-pCV5Z), which is the largest basis set for the Cl and O atoms for now, to evaluate the CV correction. Our computational results show that inclusion of this effect reduces Re by 0.00014, 0.00015 and 0.00025 nm for the three systems, respectively. To achieve high accuracy in the ab initio calculations, the scalar relativistic corrections are not negligible, although they are quite smaller than the CV corrections. In this work, the relativistic correction introduces an impact of 0.0001 nm increment on average for Re of the three systems. Comparing with previous results, either experimental determinations or theoretical predictions, it is found our data agree with those values quite well. For example, the ClO radical has been extensively examined, and the most precise value of Re is 0.156959(1) nm, which is obtained from the research of microwave spectroscopy [14]. The difference between this experimental value and our result is 0.2%. The excellent agreement provides evidence of the high accuracy of our investigation, which includes the process of extrapolating to the CBS limit with basis sets up to sextuple-zeta quality and contributions of corrections from CV correlation and relativistic effect. Besides, several computational studies also reported equilibrium geometrical parameters of ClO with the CCSD(T) [31,32], MP2 [33], MRCI [34] and B3LYP [35] methods. Among these theoretical literatures, although Re derived from Peterson et al. [32], on the CCSD(T)/CBS(TQ5) level with contributions to the total energies due to CV correlation, spin–orbit coupling effect and higher electron correlation beyond the CCSD(T) theory, as well as those obtained by Qian et al. [34], on the MRCI/CBS(DTQ) level, were believed to be more trustable than others, our result is closer to experimental values. In 1992, Gilles et al. recorded photoelectron spectra of ClO [37], which is the only experimental research to our knowledge. The agreement of Re between Gilles et al., 0.1673(8) nm, and this work is reasonable. In previous theoretical studies, calculations were performed at the MP2 [33], MP4 [39,40] and B3LYP [41] methodologies, as well as on the CCSD(T)/AVQZ [31], CCSD(T)/CBS(TQ5) [32] and MRCI/CBS(DTQ) [34] levels, respectively. As most of these calculations have been done within small- to moderate-size basis sets, our high-level computations are expected to generate more reliable results.

As to ClO+, our Re value agrees well with other theoretical estimates. Based on the excellent agreement between our prediction and experimental determination of both ClO and ClO, our calculations on ClO+ are expected to derive more precise results than previous works. As no experimental investigation concerning ClO+ has been published, to the best of our knowledge, it is impossible for us to carry out more detailed comparisons. Nevertheless, our study presents valuable information of the molecular structure of ClO+, and can serve as guidelines for further researches on such molecular ion. 3.2. Absolute energies at Re and dissociation energies For the absolute energies E at Re of each species, on the basis of the high-level computations in this work, our data are anticipated to be more reliable than previous theoretical estimates. The total energies suggested in this work are 536.40689 Eh, 536.00900 Eh and 536.49209 Eh for ClO, ClO+ and ClO, respectively. It may be noted that the energies decrease with the improvement of basis set used, and this trend is the same with that of Re. As to contributions from the CV and the relativistic effect to the total energies of all three species, corrections of the latter overweight that of the former, which is opposite to their corresponding influences to Re. To be specific, the CV corrections decrease the absolute energies E at Re 0.128 Eh of all three systems, while the relativistic corrections reduce 1.466 Eh. We should mention that our dissociation energy D0, which is a measurable molecular property, is obtained from the fitting and has been corrected by the corresponding zero-point energy. The results are listed in Table 1 for all species of interest in this work. Among the experimental investigations performed on the ClO radical, only one of them presented its dissociation energy. Coxon et al. recorded five emission bands of the A2Pi–X2Pi transitions of 35ClO isotope in the ultraviolet region [17]. The experimentally determined D0, 2.7504(4) eV, is quantitatively consistent with our result of 2.674 eV. Strictly speaking, these two values cannot be compared directly. As the isotopic effect is anticipated to have a slight impact on D0, it is safe for us to conclude that the agreement seems satisfactory. Furthermore, our result also agrees well with previous theoretical predictions. Our D0 values of the other two systems are 4.867 eV for ClO+ and 3.529 eV for ClO. For both systems, no experiment data is available. As can be seen from the table, previous calculations yielded comparable results with our data. Nevertheless, it is necessary to explain the discrepancies of D0 for ClO of both Ref. [32]

457

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 453–460 Table 3 Spectroscopic parameters of ClO, ClO+ and ClO (in cm1).

xe

Be 35

103ae

xeve

106 Drot

Be

16

37

Cl O

AV6Z CBS Exp [13] Exp [14] Exp [20] Exp [21] Exp [17] Exp [15] Exp [38]a Theory [35]c Theory [41]d Theory [32]e Theory [34]f

0.62376 0.62434 0.620525(4) 0.620501(4) 0.623459(18) – 0.623345(2) 0.62345797(4) – – – – 0.62604

6.069 6.018 – – 5.944(4) 5.93528(69) 5.5(5) 5.93571(10) – – – – 5.92037

5.77 5.64 – – 5.603(25) 5.579573(95) 5.8(4) 5.518278(60) – – – – 4.5958

1.336 1.339 2.57(53) 1.325(9) 1.37(9) – – 1.32998(7) – – – – –

Cl16O+

0.70454 0.70188 – – 0.69583

a b c d e f g h i j

106 Drot

0.61318 0.61375 0.610029(4) 0.610007(4) 0.61290 – – – – – – – –

845.28 845.42 610 – 846.45035 846.45538(132) – – – – – – –

5.916 5.865 – – 5.79626 5.78505877 – – – – – – –

5.68 5.55 – – 5.48509 5.48455(66) – – – – – – –

1.291 1.294 2.47 1.297(11) 1.2845 – – – – – – – –

1083.39 1078.07 – – –

5.834 5.198 – – –

8.36 7.29 – – –

1.132 1.131 – – –

620.66 630.74 – – – – – –

5.584 6.504 – – – – – –

3.29 4.64 – – – – – –

1.515 1.525 – – – – – –

Cl16O+

1092.70 1087.32 1139 1128.6 1041.05

5.986 5.333 – – 6.02355

8.50 7.42 12.38 – 5.9835

1.172 1.170 – – –

0.69259 0.68998 – – –

625.98 636.16 665(25) 698.02 669 – 668.6 997.21j

5.729 6.673 – – – – – 3.89349

3.34 4.72 – – – – – 3.6971

1.568 1.578 – – – – – –

0.52646 0.53333 – – – – – –

Cl16O

0.53554 0.54253 – – – 0.534 – 0.53083

xeve

37

35

AV6Z CBS Exp. [37] Theory [39]h Theory [31]i Theory [41]d Theory [32] e Theory [34]f

103ae

Cl O

852.54 852.68 616(61) – 853.764(50) 853.72446(20) 853.8(23) 853.642681(133) 844(2) b 854(3) 861 856.7 828.305

35

AV6Z CBS EXP [44] Theory [42]g Theory [34]f

xe 16

37

Cl16O

Result of the ground 2P3/2 state. Values obtained from the standard atomic weight of each element are in italic text. B3LYP/cc-pVQZ calculation. B3LYP/CBS(DTQ5) calculation. CCSD(T)/CBS calculation including contributions from several corrections, for 35ClO isotope. MRCI/CBS(DTQ) calculation. CISD/modified Gaussian-type orbitals calculation. MP4SDQ/modified Gaussian-type orbitals calculation. CCSD(T)/AVQZ calculation. We believe this value may have been misprinted instead of 697.21.

and Ref. [41] and all other studies. Value of 1.4605 eV for 35ClO, computed by utilizing the CCSD(T)/AV5Z method coupled with various of corrections by Peterson et al. [32], was the energy relative to the Cl(1S) + O(3P) dissociation channel, not the channel corresponds to its ground electronic state, which is Cl(2P) + O(2P). The analysis can also be established when interpreting result of 1.40 eV obtained by Midda and Das [41]. As an antibonding electron of neutral ClO has been lost in the ionization process, the cation has a much stronger bond than for the neutral radical, which is consistent with our prediction. Nevertheless, there also exists a disagreement for the dissociation limit of the ground state of ClO+. According to their MP4SDTQ computation, Peterson and Woods yielded the channel of Cl(2Pu) + O+(4Su) [42]. With the MRCI methodology, which is capable of treating effect of electron correlation more sufficiently, Lane and Orr-Ewing [43] and Freeman et al. [44] both support the dissociation limit of Cl+(3Pg) + O(3Pg). Although the dissociation energy was reported in Ref. [34], the dissociation channel was not identified, and no discussion was given. For further confirmation of the dissociation channels of the ground electronic states of the present systems, we have also performed computations on the ground and several low-lying excited states of both ionic species on the MRCI level. As this paper focuses on the ground states, we will not present details of their corresponding excited states here. The dissociation

channels of the ground electronic states of the neutral and ionic systems are:

ClOðX2 PÞ ! Clð2 Pu Þ þ Oð3 Pg Þ 

ClO ðX1 Rþ Þ ! Clð2 Pu Þ þ O ð2 Pu Þ

ð4Þ ð5Þ

and þ

þ

ClO ðX3 R Þ ! Cl ð3 Pg Þ þ Oð3 Pg Þ

ð6Þ

The relative large D0 of both ionic species indicates the character of electronic stability, and ensures the feasibility of performing experimental detections. Due to the lack of literatures and reference data, this work provides valuable information of the molecular structures for the present systems, especially for the ionic species, and the data can be used to reproduce accurate details of their ground electronic states. 3.3. Spectroscopic constants and vibrational energy levels To evaluate our fitting quality, the root mean square (RMS) error is introduced to evaluate the quality of our fittings, which is:

ERMS

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi #ffi u" N X 1u 2 ¼ t ðV fit  V cal Þ N i¼1

ð7Þ

458

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 453–460

Table 4 Vibrational energy levels, rotational constants and centrifugal distortion constants for ClO, ClO+ and ClO with J = 0 (in cm1). v

G(v) 35

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

106Dv

0.61808 0.61237 0.60664 0.60077 0.59504 0.58902 0.58325 0.57768 0.57205 0.56587 0.55905 0.55250 0.54628 0.53982 0.53305 0.52624 0.51925 0.51201 0.50454 0.49683 0.48882

1.336 1.349 1.335 1.353 1.349 1.344 1.345 1.294 1.334 1.431 1.433 1.417 1.439 1.474 1.507 1.538 1.565 1.606 1.658 1.716 1.775

Cl16O

417.00 1245.84 2062.19 2868.66 3663.55 4447.42 5220.55 5983.30 6737.84 7482.15 8212.27 8928.18 9631.30 10321.77 10998.79 11661.78 12310.52 12944.71 13563.82 14167.29 14754.51

G(v) 35

Cl O

420.58 1256.43 2079.61 2892.72 3694.06 4484.17 5263.40 6032.15 6792.54 7542.27 8277.39 8998.16 9706.03 10400.96 11082.10 11748.96 12401.27 13038.72 13660.75 14266.77 14856.16 37

Bv

16

1.291 1.303 1.290 1.308 1.303 1.300 1.302 1.251 1.285 1.377 1.386 1.371 1.387 1.419 1.455 1.482 1.507 1.546 1.594 1.649 1.704

Cl O

Bv

106Dv

+

531.58 1590.21 2635.01 3666.22 4683.98 5688.34 6679.35 7657.15 8621.77 9573.30 10511.81 11437.34 12349.98 13249.78 14136.81 15011.14 15872.84 16721.98 17558.66 18382.98 19195.04 37

0.60763 0.60206 0.59648 0.59076 0.58518 0.57931 0.57367 0.56823 0.56277 0.55680 0.55016 0.54375 0.53768 0.53141 0.52484 0.51822 0.51143 0.50441 0.49716 0.48968 0.48193

16

35

with Vfit and Vcal are fitted and calculated energies respectively. N is the number of fitting points. Table 2 lists our fitting parameters a1 to a12 of the M–S function and ERMS of each fitting at the AV6Z and CBS limit levels. As can be seen, ERMS for all systems are superior to chemical accuracy (1.0 kcal/mol or 349.75 cm1). According to these fitting parameters, the force constants of the systems are calculated and are presented in the same table. The spectroscopic constants of all isotopes for each system can be computed according to the fitting parameters and the equilibrium molecular constants. In Table 3, only isotopes containing 16 O, the most abundant isotope of oxygen, are tabulated, which are 35Cl16O, 37Cl16O, 35Cl16O+, 37Cl16O+, 35Cl16O and 37Cl16O. The equilibrium rotational constant Be is around 0.62 cm1, 0.70 cm1 and 0.53 cm1 for ClO, ClO+ and ClO respectively. In previous experimental researches, the microwave studies of Ref. [13] and Ref. [14], performed in 1969 and in 1978 separately, derived values of Be and xe of 35Cl16O to be 0.6205 cm1 and 616 cm1, while the infrared [20,21], ultraviolet [17] as well as a later microwave [15] investigation suggested these values to be 0.6234 cm1 and 853.7 cm1, which are in good accord with our results. In the case of the 37Cl16O isotope, the discrepancies of the constants between Ref. [13] and Ref. [14] and other

16

Cl O

0.70205 0.69603 0.68997 0.68389 0.67779 0.67166 0.66554 0.65941 0.65327 0.64714 0.64100 0.63487 0.62874 0.62261 0.61649 0.61038 0.60427 0.59817 0.59208 0.58601 0.57997

1.222 1.227 1.231 1.234 1.237 1.241 1.243 1.246 1.249 1.251 1.255 1.257 1.261 1.263 1.267 1.270 1.273 1.276 1.278 1.280 1.282

Cl16O+

527.04 1576.77 2612.91 3635.69 4645.24 5641.61 6624.85 7595.10 8552.40 9496.82 10428.44 11347.29 12253.47 13147.01 14027.99 14896.48 15752.55 16596.26 17427.70 18246.97 19054.18

G(v)

Bv

106Dv

0.53002 0.52612 0.52197 0.51759 0.51303 0.50833 0.50354 0.49871 0.49385 0.48899 0.48416 0.47936 0.47461 0.46991 0.46527 0.46069 0.45616 0.45169 0.44725 0.44286 0.43851

1.524 1.530 1.521 1.503 1.481 1.459 1.438 1.418 1.400 1.383 1.367 1.352 1.337 1.322 1.308 1.295 1.283 1.273 1.263 1.253 1.245

0.52105 0.51725 0.51321 0.50895 0.50450 0.49993 0.49527 0.49056 0.48582 0.48108 0.47637 0.47169 0.46706 0.46247 0.45795 0.45347 0.44905 0.44468 0.44035 0.43607 0.43182

1.473 1.479 1.470 1.453 1.432 1.410 1.390 1.372 1.354 1.338 1.323 1.308 1.293 1.279 1.266 1.253 1.242 1.232 1.222 1.213 1.204



313.77 936.20 1551.64 2160.76 2763.92 3361.25 3952.79 4538.50 5118.31 5692.18 6260.08 6822.02 7378.01 7928.11 8472.37 9010.84 9543.58 10070.61 10591.97 11107.69 11617.77 37

Cl16O

0.69018 0.68430 0.67840 0.67247 0.66653 0.66056 0.65459 0.64861 0.64263 0.63665 0.63067 0.62470 0.61872 0.61275 0.60678 0.60082 0.59486 0.58892 0.58298 0.57707 0.57117

1.181 1.186 1.189 1.193 1.196 1.199 1.201 1.204 1.207 1.209 1.212 1.215 1.218 1.221 1.224 1.227 1.230 1.233 1.235 1.237 1.238

311.11 928.29 1538.60 2142.69 2740.91 3333.40 3920.20 4501.26 5076.54 5645.98 6209.55 6767.25 7319.12 7865.17 8405.49 8940.11 9469.08 9992.44 10510.22 11022.45 11529.14

experimental studies, including this work, are similar to the situation of 35Cl16O. Our constants of ae, xeve and Drot are also excellent agree with previous experimental works. Apparently, prediction of Be and xe in Ref. [34] deviates away from all above mentioned data, which may attribute to the fact that (1) the largest basis set used in their study was AVQZ although the CBS extrapolation technique was utilized and (2) the results were derived from the standard atomic weight of each element. In general, our values of ClO radical are expected to be trustable results, which make us confidence about the reliability of our data for both ClO+ and ClO. When the ClO molecule loses (captures) an electron and becomes the corresponding cation (anion) species, shortening of 6% (lengthening of 7%) of Re between O and Cl atoms leads to an increment of 12% (decrement of 13%) of Be, and significant alteration of bond strength can further be concluded from the large percentage variations of xe. With the aid of the LEVEL program, the vibrational energy levels of the ground electronic states (with the rotational quantum number J = 0) for all isotopes of ClO, ClO+ and ClO are obtained by solving the one-dimensional Schrödinger equation of nuclear motion numerically. For the sake of simplicity, energy levels G(v), rotational constants Bv and quadruple centrifugal distortion constants

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 453–460 Table 5 Ionization potentials and electron affinities of ClO and vertical detachment energies of ClO (in eV).

This work Exp. [37] Exp. [38] Theory [31]a Theory [32]b Theory [33]c Theory [34]d Theory [39]e Theory [41]f Theory [42]g

AIP

VIP

AEA

VEA

VDE

10.843 – – – – 10.78 – – – 11.188

11.039 – – – – – – – – –

2.342 2.266(2) 2.2775(13) 2.29 2.260138 2.35 2.1430538 2.16 2.24 –

2.202 – – – – – – – – –

2.489 – – 2.39 – – – – – –

459

theoretical researches and can be conducive to extending our understanding on characteristics of these systems. Acknowledgments The authors gratefully acknowledge Dr. Ming-Jie Wan of Yibin University for useful discussions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 11304023 and 11447172) and by the Yangtze Youth Talents Fund (S. Li). References

a

MP4SDTQ/modified Gaussian-type orbitals calculation. CCSD(T)/CBS calculation including contributions from several corrections, for ClO isotope. c MP2/6-31G(d) calculation. d MRCI/CBS(DTQ) calculation. e MP4SDQ/modified Gaussian-type orbitals calculation. f B3LYP/CBS(DTQ5) calculation. g CISD/modified Gaussian-type orbitals calculation. b

35

Dv of the 16O-containing systems with the vibrational quantum number v up to 20 are listed in Table 4. Our investigation presents useful information for further experimental and theoretical researches such as studies of isotope identification and spectrum assignment of these systems. 3.4. Ionization potentials, electron affinities and vertical detachment energies The adiabatic ionization potential (AIP), vertical ionization potential (VIP), adiabatic electron affinity (AEA) and vertical electron affinity (VEA) of the ClO radical, as well as vertical detachment energy (VDE) of the corresponding negative molecular ion, have been determined by using the RCCSD(T) [58,59] method coupled with the AV6Z basis set. The results are collected in Table 5 with the inclusion of several literature data. Our AEA is 2.342 eV, which agrees quite well with experimental measurements given in Ref. [37] and Ref. [38], with slight differences of less than 3%, and all other theoretical predictions. Both AIP and VDE are consistent with literature data available. Values of VIP and VEA in this work are reported for the first time, and the VIP can be used to predict and to assist detecting and analyzing the photoelectron spectrum.

4. Conclusion In this paper, we have carried out a series of high-level calculations on ClO, ClO+ and ClO species with the MRCI method. Computation results with the aug-cc-pVXZ (X = Q, 5, 6) basis sets are extrapolated to the complete basis set limit and corrections of core-valence correlation and relativistic effect are also considered. The equilibrium geometrical structures, potential energy curves and equilibrium spectroscopic constants of the ground electronic states for all three systems have been studied. The vibrational energy levels of the ground state with J = 0 and molecular constants for each of the levels are presented. Comparisons of equilibrium geometrical parameters and spectroscopic constants between our results and previously reported experimental ones indicate reasonable agreements, which further suggest results of our work are expected to be trustworthy predictions. The information derives from this work, which are believed to supplement basic molecular information of the neutral and ionic ClO species, especially of ClO+ and ClO, can support further experimental or

[1] Y. Fukui, H. Ogawa, K.C. Xiao, Y. Iwasaka, H. Nakane, T. Nagahama, Adv. Space Res. 26 (2000) 975–978. [2] Y. Bedjanian, G. Poulet, Chem. Rev. 103 (2003) 4639–4656. [3] U. Platt, C. Janssen, Faraday Discuss. 100 (1995) 175–198. [4] A. Saiz-Lopez, R. von Glasow, Chem. Soc. Rev. 41 (2012) 6448–6472. [5] W.R. Simpson, S.S. Brown, A. Saiz-Lopez, J.A. Thornton, R. von Glasow, Chem. Rev. 115 (2015) 4035–4062. [6] S. Li, R. Zheng, S.J. Chen, D.S. Zhu, Q.C. Fan, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 133 (2014) 735–740. [7] G. Pannetier, A.G. Gaydon, Nature 161 (1948) 242–243. [8] G. Porter, Proc. R. Soc. A 200 (1950) 284–300. [9] G. Porter, Discuss. Faraday Soc. 9 (1950) 60–69. [10] R.A. Durie, D.A. Ramsay, Can. J. Phys. 36 (1958) 35–53. [11] A. Carrington, P.N. Dyer, D.H. Levy, J. Chem. Phys. 47 (1967) 1756–1763. [12] T. Amano, E. Hirota, Y. Morino, J. Mol. Spectrosc. 27 (1968) 257–265. [13] T. Amano, S. Saito, E. Hirota, Y. Morino, J. Mol. Spectrosc. 30 (1969) 275–289. [14] R.K. Kakar, E.A. Cohen, M. Geller, J. Mol. Spectrosc. 70 (1978) 243–256. [15] B.J. Drouin, C.E. Miller, E.A. Cohen, G. Wagner, M. Birk, J. Mol. Spectrosc. 207 (2001) 4–9. [16] N. Basco, R.D. Morse, J. Mol. Spectrosc. 45 (1973) 35–45. [17] J.A. Coxon, W.E. Jones, E.G. Skolnik, Can. J. Phys. 54 (1976) 1043–1052. [18] A. Loewenschuss, J.C. Miller, L. Andrews, J. Mol. Spectrosc. 80 (1980) 351–362. [19] R.S. Lowe, A.R.W. McKellar, J. Mol. Spectrosc. 79 (1980) 424–431. [20] A.G. Maki, F.J. Lovas, W.B. Olson, J. Mol. Spectrosc. 92 (1982) 410–418. [21] J.B. Burkholder, P.D. Hammer, C.J. Howard, J. Mol. Spectrosc. 124 (1987) 139– 161. [22] M.T. Duignan, J.W. Hudgens, J. Chem. Phys. 82 (1985) 4426–4433. [23] M.J. Cooper, T. Diez-Rojo, L.J. Rogers, C.M. Western, M.N.R. Ashfold, J.W. Hudgens, Chem. Phys. Lett. 272 (1997) 232–238. [24] S. Baumgärtel, K.-H. Gericke, Chem. Phys. Lett. 227 (1994) 461–466. [25] N.P.L. Wales, W.J. Buma, C.A. de Lange, Chem. Phys. Lett. 259 (1996) 213–218. [26] S. Schmidt, T. Benter, R.N. Schindler, Chem. Phys. Lett. 282 (1998) 292–298. [27] P. Zou, H. Kim, S.W. North, J. Chem. Phys. 116 (2002) 4176–4183. [28] H. Kim, J. Park, T.C. Niday, S.W. North, J. Chem. Phys. 123 (2005) 174303. [29] H. Kim, K.S. Dooley, G.C. Groenenboom, S.W. North, Phys. Chem. Chem. Phys. 8 (2006) 2964–2971. [30] K.S. Dooley, M.P. Grubb, J. Geidosch, M.A. van Beek, G.C. Groenenboom, S.W. North, Phys. Chem. Chem. Phys. 11 (2009) 4770–4776. [31] S. Kim, Y.J. Kim, C.H. Shin, B.J. Mhin, T.D. Crawford, J. Chem. Phys. 117 (2002) 9703–9709. [32] K.A. Peterson, B.C. Shepler, D. Figgen, H. Stoll, J. Phys. Chem. A 110 (2006) 13877–13883. [33] N.L. Ma, Y.S. Cheung, C.Y. Ng, W.K. Li, Mol. Phys. 91 (1997) 495–501. [34] Q. Qian, C.L. Yang, F. Gao, X.Y. Zhang, Acta Phys. Sin. 56 (2007) 4420–4427. [35] R. Janoschek, J. Mol. Struct. (Theochem) 423 (1998) 219–224. [36] R.B. Singh, D.K. Rai, J. Quant. Spectrosc. Radiat. Transfer 5 (1965) 723–727. [37] M.K. Gilles, M.L. Polak, W.C. Lineberger, J. Chem. Phys. 96 (1992) 8012–8020. [38] V. Distelrath, U. Boesl, Faraday Discuss. 115 (2000) 161–174. [39] K.A. Peterson, R.C. Woods, J. Chem. Phys. 92 (1990) 7412–7417. [40] S.T. Howard, Mol. Phys. 85 (1995) 395–406. [41] S. Midda, A.K. Das, J. Mol. Struct. (Theochem) 713 (2005) 101–106. [42] K.A. Peterson, R.C. Woods, J. Chem. Phys. 93 (1990) 1876–1888. [43] I.C. Lane, A.J. Orr-Ewing, Mol. Phys. 98 (2000) 793–806. [44] C.D. Freeman, M.P. Grubb, I.C. Lane, S.W. North, Chem. Phys. 408 (2012) 43–49. [45] H.-J. Werner, P.J. Knowles, G. Knizia, F.R. Manby, M. Schütz, P. Celani, T. Korona, R. Lindh, A. Mitrushenkov, G. Rauhut, K.R. Shamasundar, T.B. Adler, R.D. Amos, A. Bernhardsson, A. Berning, D.L. Cooper, M.J.O. Deegan, A.J. Dobbyn, F. Eckert, E. Goll, C. Hampel, A. Hesselmann, G. Hetzer, T. Hrenar, G. Jansen, C. Köppl, Y. Liu, A.W. Lloyd, R.A. Mata, A.J. May, S.J. McNicholas, W. Meyer, M.E. Mura, A. Nicklass, D.P.O’Neill, P. Palmieri, D. Peng, K. Pflüger, R. Pitzer, M. Reiher, T. Shiozaki, H. Stoll, A.J. Stone, R. Tarroni, T. Thorsteinsson, and M. Wang, MOLPRO, version 2012.1, a package of ab initio programs, see . [46] H.-J. Werner, P.J. Knowles, J. Chem. Phys. 89 (1988) 5803–5814. [47] H.-J. Werner, P.J. Knowles, J. Chem. Phys. 82 (1985) 5053–5063. [48] K.A. Peterson, D.E. Woon, T.H. Dunning Jr., J. Chem. Phys. 100 (1994) 7410– 7415.

460 [49] [50] [51] [52] [53] [54] [55]

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 453–460 D. Feller, J. Chem. Phys. 96 (1992) 6104–6114. M. Douglas, N.M. Kroll, Ann. Phys. 82 (1974) 89–155. B.A. Hess, Phys. Rev. A 33 (1986) 3742–3748. J.N. Murrell, K.S. Sorbie, J. Chem. Soc., Faraday Trans. 70 (1974) 1552–1556. S. Li, L.B. Han, S.J. Chen, C.X. Duan, Acta Phys. Sin. 62 (2013) 113102. S. Li, S.J. Chen, D.S. Zhu, J.J. Wei, Acta Phys. Chim. Sin. 29 (2013) 737–744. S. Li, S.J. Chen, D.S. Zhu, Q.C. Fan, Comput, Theor. Chem. 1017 (2013) 136–143.

[56] S. Li, R. Zheng, S.J. Chen, Q.C. Fan, Mol. Phys. 113 (2015) 1433–1441. [57] R.J. Le Roy, Level 8.0: A Computer Program for Solving the Radial Schrödinger Equation for Bound and Quasibound Levels, University of Waterloo Chemical Physics Research Report CP-663, see . [58] M.J.O. Deegan, P.J. Knowles, Chem. Phys. Lett. 227 (1994) 321–326. [59] P.J. Knowles, C. Hampel, H.-J. Werner, J. Chem. Phys. 99 (1993) 5219–5227.