Noble-Gas-Inserted Fluoro(sulphido)boron (FNgBS, Ng = Ar, Kr, and

Subscriber access provided by CARLETON UNIVERSITY

Article

Noble Gas Inserted Fluoro(sulphido)boron (FNgBS, Ng = Ar, Kr, and Xe): A Theoretical Prediction Ayan Ghosh, Sourav Dey, Debashree Manna, and Tapan K. Ghanty J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp512520y • Publication Date (Web): 30 Apr 2015 Downloaded from http://pubs.acs.org on May 5, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Noble Gas Inserted Fluoro(sulphido)boron (FNgBS, Ng = Ar, Kr, and Xe): A Theoretical Prediction

Ayan Ghosh#,‡, Sourav Dey§,‡, Debashree Manna†, and Tapan K. Ghanty†,* #

Laser and Plasma Technology Division, Beam Technology Development Group, Bhabha Atomic Research Centre, Mumbai 400 085, INDIA. § Ramakrishna Mission Vidyamandira, Belur Math, West Bengal, INDIA. † Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400 085, INDIA.

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The possibility of the existence of a new series of neutral noble gas compound, FNgBS (where Ng = Ar, Kr, Xe) is explored theoretically through insertion of a Ng atom into the fluoroborosulphide molecule (FBS). Second order Møller–Plesset perturbation theory, density functional theory, and coupled cluster theory based methods have been employed to predict the structure, stability, harmonic vibrational frequencies, and charge distribution of FNgBS molecules. Through energetics study, it has been found that the molecules could dissociate into global minima products (Ng + FBS) on the respective singlet potential energy surface via a unimolecular dissociation channel. However, the sufficiently large activation energy barriers provide enough kinetic stability to the predicted molecules, which in turn prevent them to dissociate into the global minima products. Moreover, the FNgBS species are thermodynamically stable owing to very high positive energies with respect to other two 2–body dissociation channels leading to FNg + BS and F- + NgBS+, and two 3–body dissociation channels corresponding to the dissociation into F + Ng + BS and F- + Ng + BS+. Further, the Mulliken and NBO charge analysis together with the AIM results reveal that the Ng–B bond is more of covalent in nature, whereas the F–Ng bond is predominantly ionic in character. Thus, these compounds can be better represented as F-[NgBS]+. This fact is also supported by the detail analysis of bond length, bond dissociation energy and stretching force constant values. All the calculated results reported in this work clearly indicate that it might be possible to prepare and characterize the FNgBS molecules in cryogenic environment through matrix isolation technique by using a mixture of OCS/BF3 in presence of large quantity of noble gas under suitable experimental conditions.

Keywords: Noble gas; Insertion compound; FBS; Ab initio calculations; Structure and stability; Charge distribution; AIM properties; Vibrational frequencies *

Author to whom correspondence should be addressed. Electronic mail: [email protected].

‡

These authors contributed equally.

2


Page 2 of 31

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction The noble gas elements were first discovered by Ramsay and co-workers in the 1890s and since then they have been a standard example for studying the chemical inertness of a system. After many unsuccessful attempts, the first noble gas compound, XePtF6 had been synthesized by Bartlett in 1962.1 In the last decade various molecules consisting of one or more noble gas atoms2-32 have been predicted through experimental and theoretical techniques. The discovery of the first covalently bonded compound of argon, HArF, prepared experimentally by Räsänen and co-workers2 in the year 2000 has motivated researchers to predict various new noble gas insertion compounds. Subsequently, extensive amount of work have been carried out to provide an in-depth insight into the nature of chemical bonds and to enhance the general understanding about metastable molecules involving noble gas atom. Various ionic or neutral insertion molecules of noble gas atoms with environmentally important species such as HOX12 (X = Cl, Br, F), H3O+25 and species of astronomical significance, like HCO+22, HCS+23, HN2+24 etc. have been theoretically investigated using various computational methods. Of late, one of the noble gas insertion molecules, HXeOBr has been successfully prepared and characterized using IR spectroscopic technique by Khriachtchev et al,32 which was theoretically predicted by our group earlier.12 In the year 2005, Hu and his group had theoretically predicted a series of noble gas insertion compound of the type of FNgBO27 (where Ng = Ar, Kr and Xe). Subsequently, FNgBN–28 species, which are isoelectronic with FNgBO molecules, have been reported by Grandinetti and co-workers. Very recently, FNgBNR molecules with R = H, CH3, CCH, CHCH2, F, and OH have also been investigated.29 Motivated from these theoretical findings and experimental study of FBS through microwave as well as photoelectron spectroscopic techniques,33-40 we present here the theoretical investigation of a novel noble gas insertion molecules of the type FNgBS (where Ng = Ar, Kr and Xe). Very recently, we have predicted a new noble gas inserted thioformyl cation HNgCS+, where a detail comparison has been made between the HNgCO+22 and HNgCS+23 ions with respect to their structure, stability, spectroscopic properties and charge distribution analysis. It has been found that NgC interaction is much stronger in HNgCS+23 ions as compared to that in HNgCO+22 species. It is due to the larger size, higher polarizibily and smaller electronegativity of the sulphur atom than the oxygen atom, resulting into stronger interaction of noble gas with the carbon atom adjacent to the sulphur atom in HNgCS+ ions. Since, the earlier work was associated with the ionic 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

species; here we are keen to understand the nature of bonding present in the neutral molecule, FNgBS, and compare with the previously reported FNgBO species.27 Hence, we present here the structural, energetics, charge distribution and spectroscopic analysis of a series of noble gas insertion compound, FNgBS in a systematic way. To the best of our knowledge there is no report on FNgBS molecule in the literature till date. 2. Computational Methodology In this work, all the calculations have been performed through ab initio molecular orbital and density functional theory based methods using GAMESS41 and MOLPRO 201242 programs. The minima and the transition state geometries of FNgBS molecules have been optimized using the MP243, DFT with Becke 3parameter exchange and Lee−Yang−Parr correlation (B3LYP)44,45, and CCSD(T)46 based methods. The structural optimizations have been performed using the C∞v point group for the minima and Cs point group for the transition state structure on the singlet potential energy surface. We have used the energy adjusted Stuttgart effective core potentials47 (ECPs) consisting of 28 and 46 core electrons for the Kr and Xe atom, respectively, and the corresponding valence only (6s6p1d1f)/[4s4p1d1f] basis sets. The standard split valence basis sets with polarization functions, viz., 6311++G(2d,2p) have been employed for the B, F, S, and Ar atoms in the B3LYP and MP2 calculations. This grouping of basis sets has been denoted as B1. It may be noted that similar combination of basis sets was used earlier by Lignell et al.48 while discussing the reliability of various theoretical methods, viz., B3LYP, MP2 and CCSD(T), in the prediction of noble gas hydrides. In the present work, the CCSD(T) calculations have been done using the same ECPs along with the (6s6p1d1f)/[4s4p1d1f] basis sets for the Kr and Xe atoms, and the augccpVTZ basis sets for the remaining elements. This combination of basis set has been designated as B2. Moreover, we have considered valence only augccpVTZ−PP basis sets for the Kr and Xe atoms with 10 and 28 core electrons,49 respectively, and the augccpVTZ basis sets for the B, F, S, and Ar atoms for the B3LYP, MP2 and CCSD(T) calculations. We have denoted this combination of basis sets as B3. Additionally, we have used augccpVQZ−PP basis set for the Xe atom with 28 core electrons, and the augccpVQZ basis sets for the B, F and S atoms, which is represented as B4. Recently, a benchmark study on the prediction of bond lengths and bond energies of various noble gas compounds has been reported by Hu and co-workers,50 by using several density functionals and various basis sets including aug−cc−pVTZ and aug−cc−pVQZ, 4


Page 4 of 31

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where the complete basis set (CBS) limit extrapolated CCSD(T) energies have been considered as reference values. Vibrational analysis, Mulliken population and NBO analysis have also been carried out for all the species using the B3LYP and MP2 methods with B1 basis sets, and CCSD(T) method with B2 basis sets. Also the AIM (Atoms-in-Molecule) analysis has been applied to provide a clear perception of the nature of chemical bonding exists between different atoms quantitatively employing MP2/B1 and B3LYP/B1 methods. 3. Results and Discussions 3.1. Structural Parameters of FNgBS The noble gas inserted neutral FBS molecule (FNgBS) exhibits a linear structure for the minima with Cv symmetry and a non–linear planar structure for the transition state geometry with Cs symmetry on their respective singlet potential energy surfaces. Figure 1 depicts the graphical representation of the optimized minima and transition state structure of all the FNgBS molecules with the structural parameters obtained by MP2/B1, CCSD(T)/B2, and CCSD(T)/B3 levels. For the purpose of comparison all the geometrical parameters of optimized structures of FNgBS molecules as obtained using B3LYP/B1, B3LYP/B3, MP2/B1, MP2/B3, CCSD(T)/B2 and CCSD(T)/B3 levels are presented in Tables S1 and S2 for the minima and the transition state geometries, respectively (Supporting Information). In general, the B3LYP/B3 calculated bond length values are found to be slightly larger as compared to the corresponding CCSD(T)/B3 values, and the corresponding MP2/B3 values are slightly smaller. However, in most cases, the difference in bond length is < 0.01 Å as obtained using MP2/B1, CCSD(T)/B2, and CCSD(T)/B3 methods for both F−Ng and Ng−B bonds (Figure 1). Now, it is of interest to analyze the F−Ng bond distance in FNgBS molecules. The F–Ng bond lengths are 2.028 [2.028], 2.054 [2.056] and 2.127 [2.126] Å in FArBS, FKrBS and FXeBS species, respectively, as calculated using CCSD(T)/B2 [CCSD(T)/B3] method. Thus, the CCSD(T)/B2 and CCSD(T)/B3 calculated F−Ng bond lengths are found to be almost the same. Since the optimized structural parameters calculated using the CCSD(T) method are considered better than the corresponding MP2 or B3LYP values, we henceforth refer to the CCSD(T)/B2 values, unless otherwise stated. Now it is worth comparing these bond lengths with the F−Ng bond lengths in FNgBO molecules, which are given in Table S3 (Supporting Information). Using the CCSD(T)/B2 method for calculation, we found that the FNg bond lengths are 1.989, 2.023 and 2.103 Å for FArBO, FKrBO, and FXeBO species, respectively. There is a 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

slight increase in F–Ng bond length values in going from FNgBO to FNgBS species. Increase in the FNg bond length values in both FNgBO and FNgBS species along the Ar–Kr–Xe series can be attributed to the increase in the size of the noble gas atom. The CCSD(T)/B2 [CCSD(T)/B3] computed NgB bond length values are 1.806 [1.806], 1.954 [1.968] and 2.160 [2.168] Å in FArBS, FKrBS, and FXeBS species, respectively. Now, it is interesting to compare the NgB bond length in FNgBS with the previously investigated systems likes FNgBO27 and ionic FNgBN28 systems. The CCSD(T)/B2 calculated NgB bond lengths are 1.828, 1.966, 2.169 Å along the series ArKrXe, respectively, in FNgBO species, and the corresponding values in FNgBN are 1.820, 1.961, and 2.153 Å. From the above results, it is obvious that the NgB bond in FNgBS is almost the same (very slight smaller side) as compared to the NgB bonds present in the FNgBO and FNgBN systems. The calculated Ng–B bond lengths in FNgBS are also found to be comparable to that in the FNgBNR systems (R = H, CH3, CCH, CHCH2, F, and OH) reported recently.29 Periodic variation of chemical properties of elements along a particular period or group in the periodic table has always been fascinating to chemists. Therefore, we have been motivated to compare the NgB bond lengths in the FNgBS molecules with the Ng–X (X = B, C, N) bond lengths for some noble gas inserted cationic systems (HNgBF+, HNgCO+, HNgCS+ and HNgN2+) reported23-26 recently by us in a systematic and unified way, using the same level of theory and the same basis sets. The CCSD(T)/B2 computed Ng–B bond lengths are 2.943, 2.980, 3.090 Å in HNgBF+, the Ng–C bond lengths are 2.911, 3.068, 3.124 and 2.725, 2.755, 2.872 Å in HNgCO+ and HNgCS+, respectively, and the Ng–N bond lengths are 2.841, 2.922, 3.093 Å in HNgN2+ for Ng = Ar, Kr and Xe, respectively. Thus, the corresponding Ng–X bond lengths are found to be greater than the Ng–B bond lengths in FNgBS molecules, even though the covalent radius value of boron is the maximum among boron, carbon and nitrogen.51 It clearly indicates towards the fact that the Ng–B bond is stronger in FNgBS molecules. Here it may be noted that the NgC bond length is decreased considerably when O atom is replaced with S atom in HNgCO+ species. However, the difference in the NgB bond length in FNgBO and FNgBS systems is rather negligible. All the trends in the change in bond lengths indicate that the NgB bonds in FNgBO and FNgBS systems are strong and rigid, and remain unperturbed when O atom is replaced with S atom in FNgBO species, unlike in the case of HNgCO+ species. As far as the B–S bond is considered the bond length is almost similar

6


Page 6 of 31

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for all the FNgBS species (within the range of 1.602–1.611 Å), however, is decreased slightly from the original B–S bond length value in the FBS (1.622 Å) molecule. Following the earlier works of Gerry and co-workers30,31 we would like to analyze the FNg and NgB bond lengths with respect to two limits: covalent limits and van der Waals limits, denoted respectively as Rcov and RvdW. For an XY bond these limits can be defined as, Rcov = rcov (X) + rcov(Y) and RvdW = rvdW(X) + rvdW(Y), where rcov and rvdW refers to covalent and van der Waals radius of an atom, respectively. The standard values51-53 of these radii have been taken from the literature to calculate the respective limits. The covalent limits of the F–Ng bond lengths are 1.63, 1.73 and 1.97 Å and the corresponding van der Waals limits are 3.23, 3.49 and 3.63 Å for Ng = Ar, Kr and Xe, respectively. The covalent limit of the NgB bond lengths are 1.9, 2.0 and 2.24 Å for Ar–Kr–Xe, respectively, and the corresponding van der Waals limits are 3.67, 3.93 and 4.07 Å, respectively. Thus, the F–Ng bond lengths in both FNgBS and FNgBO are slightly larger than the covalent limit and deviate considerably from that of the van der Waals limit. However, it is important to note that the Ng–B bond lengths in both the series are slightly smaller than the corresponding covalent limits. It indicates that the Ng–B bond is a relatively strong chemical bond whereas F–Ng bond is somewhat weaker than that of a covalent bond but considerably stronger than just van der Waals interaction. In fact, it is to be noted that the NgB bond lengths are close to the corresponding sum of the double bond covalent radii,54 which indicate that these systems might have a partial double bond character. For all the predicted FNgBS molecules, the bond lengths and bond angles are changed considerably in going from the minima state to the transition state structure. The FNg bond elongates by an amount of ~0.2 Å for Ng = Ar, Kr and Xe. However, it is interesting to note that the NgB bond contracts by an amount of ~0.1 Å in the transition state, where the contraction increases slightly along the series Ar–KrXe. The B–S bond length remains almost the same in all the transition state structures. Nevertheless, the FNgB angle also changes from 180 degree to 109, 101.6 and 99.5 degree in the transition state of FArBS, FKrBS, and FXeBS species, respectively, whereas there is not much change in the NgBS bond angle. This trend in the decrease in FNgB bond angle from FArBS to FXeBS may be attributed to the increase in size while going from Ar to Xe. 3.2. Energetics and Kinetic Study 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

It is well known that the study of molecular energetics is the most important part in the prediction of new compounds to ascertain their stability. Thus, in order to assess the stability of FNgBS molecules, following most plausible unimolecular dissociation channels have been considered, and the corresponding dissociation energies are reported in Table 1.

FNgBS



Ng + FBS

(1)

FNg + BS

(2)

F + NgBS+

(3)

F + Ng + BS

(4)

F + Ng + BS+

(5)

Following the recent benchmark study,50 we have considered the CCSD(T)/B3 computed energy values throughout the text unless otherwise mentioned. Three 2body dissociation channels and two 3body dissociation channels have been considered here in order to determine the thermodynamic stability of the FNgBS species. The first dissociation channel leads to the global minima products and the other channels lead to the local minima products on the singlet potential energy surface. The CCSD(T)/B3 calculated dissociation energies for the first 2body dissociation pathway are – 659.9, –569.5, –469.4 kJ mol1 along the series ArKrXe, which signifies that the predicted FNgBS molecules are thermodynamically unstable in comparison with the global minima products (FBS + Ng). The second 2body dissociation pathway has positive dissociation energy values viz. 24.0, 114.1, 208.5 kJ mol1 for FArBS, FKrBS, and FXeBS, respectively, which indicates a greater stability of FNgBS molecules as compared to the dissociated products (FNg + BS) of channel (2). The predicted FNgBS molecules are thermodynamically stable with respect to the other 2body dissociation channel, (F + NgBS+) by 571.9, 615.8, and 655.0 kJ mol−1 along the Ar–Kr–Xe series, respectively. The endothermicity of the 3body dissociation channel (4) illustrates that the predicted species are more stable than the dissociated products (F + Ng + BS) by an amount of 25.3, 115.8, and 215.8 kJ mol−1 for FArBS, FKrBS, and FXeBS species, respectively. Similarly, a very high positive energy values have been found for another 3body dissociation channel (F + Ng + BS+) where the calculated values are 785.2, 875.6, and 975.7 kJ mol−1 for FArBS, FKrBS, and 8


Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FXeBS species respectively. Furthermore, we have calculated the dissociation energies of the predicted FXeBS molecule for all probable dissociation channels with respect to the singlet state optimized structure using CCSD(T) method with augccpVQZPP basis set and reported in Table 1. The results obtained by this level of theory agree very well with the corresponding CCSD(T)/B3 results, with a deviation of only ~4 kJ mol−1. Now it is imperative to calculate the barrier height of the first dissociation pathway to measure the kinetic stability of the insertion molecules. The barrier height (without zero-point energy correction) with respect to the FNgBS  Ng + FBS comes out to be 60.8, 101.6, 132.2 kJ mol1 along the ArKrXe series of FNgBS species at CCSD(T)/B3 level calculation. On the other hand, the barrier height corresponding to FNgBO  Ng + FBO dissociation are found to be 76.6, 115.5, and 147.3 kJ mol–1 for FArBO, FKrBO, and FXeBO molecules respectively at CCSD(T)/B3 level of theory.27 From the above results, it is evident that the barrier heights are almost comparable for both FNgBO and FNgBS molecules. It may be noted here that the calculated barrier heights in case of B3LYP is greater than that in MP2 or CCSD(T), except in the case of FXeBS. Nevertheless, one must take into account the zero-point energy (ZPE) correction to evaluate more accurate values of barrier heights. The zeropoint energy correction parameter comes out to be ~2.0 kJ mol1 at the B3LYP/B1 and MP2/B1 levels of theory and the corrected barrier height remains high enough to provide sufficient kinetic stability to the FNgBS species. Hence, from the above discussion it is clear that the predicted neutral FNgBS molecules are metastable in nature since they are kinetically stable with respect to the global minima products and thermodynamically stable with respect to other possible 2body and 3body dissociation channels. Therefore, it might be possible to prepare and characterize these predicted neutral FNgBS molecules in cryogenic conditions through matrix isolation techniques. In this context, it is important to study the singlet-triplet energy gaps in order to ascertain the stability of the predicted FNgBS molecules in the ground singlet state. The singlet–triplet energy gaps have been calculated with the singlet state optimized structure using CCSD(T) methods with B2 and B3 basis sets, and listed in Table 2. The first triplet state was found to be considerably higher (>400 kJ mol-1) in energy than the corresponding ground singlet state for all the predicted FNgBS molecules. These significantly higher S–T energy gaps clearly indicate that even at a very low temperature the intersystem crossing would not take place. In addition, we have also calculated the ST crossing energy employing CCSD(T)/B2 level of theory for FNgBS molecules by taking the optimized structure of the singlet FNgBS molecules at various B=S distances. It has been found 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

that the ST crossing has taken place at a distance of 2.626, 2.688, and 2.763 Å along the B=S coordinates for FArBS, FKrBS, and FXeBS molecules and the corresponding crossing energies with respect to singlet minima state of the concerned molecules are 542.3, 569.8, and 593.2 kJ mol– 1

.

3.3. Charge Distribution Analysis of FNgBS The analysis of charge distribution is very essential to get an idea about the nature of bonding between the constituent atoms or fragments in a molecule. The partial atomic charges of FNgBS calculated at the MP2/B1 and B3LYP/B1 levels obtained from the Mulliken population analysis have been given in Table 3. The charge computed using both the methods is very similar except in few cases. However here we shall discuss the MP2/B1 calculated partial charges in detail, although the partial atomic charges obtained for FNgBS molecules by both MP2 and B3LYP methods are quite comparable. After the insertion of the noble gas atom there has been a significant redistribution of charges on fluorine, boron and sulphur (denoted respectively as qF, qB and qS) as against the same in the FBS molecule. The qF has become more negative after molecule formation and the value change from –0.074 in FBS to –0.698, –0.679, and –0.607 in FArBS, FKrBS, and FXeBS species, respectively. However there is a reasonable decrease in the positive electronic charge on the B atom, and qB decreases from 0.196 to 0.113, 0.186 and 0.194 along the ArKrXe series in FNgBS compounds. The value of qS changes from –0.122 in FBS to 0.135 in FArBS, 0.204 in FKrBS and 0.011 in FXeBS. The noble gas atom possesses partial positive charge in the FNgBS molecules and the values are 0.450, 0.662 and 0.812 for FArBS, FKrBS, and FXeBS species, respectively. Now, it is worthwhile to mention that the total accumulated charges on NgBS fragment are 0.698, 0.679, and 0.607 for FArBS, FKrBS, and FXeBS species, respectively. It indicates that substantial charge transfer has taken place after the insertion of a noble gas atom into the neutral FBS molecule. Thus, FNgBS species can be represented as the ionic configuration, F(NgBS)+. For the purpose of comparison we have also included the atomic charges on the FNgBO molecules in Table S4 (Supporting Information), which reveals that charge values for the F and Ng atoms are very close to each other in the FNgBS and FNgBO molecules. However, charge on the boron and the oxygen/sulphur atom differs due to the electronegativity difference between the oxygen and the sulphur atoms. In fact, charge on the oxygen atom in FNgBO molecules is found to be always negative; however, the charge value on the sulphur atom in FNgBS 10


Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


molecules is always positive, except in FXeBS (Table S4, Supporting Information). This trend indicates that the BS bond is rather more covalent as compared to the BO bond in FngBS and FNgBO molecules, respectively. At the transition state the qF values increase as compared to that in the corresponding minima structures of the FNgBS species and the values are –0.861, –0.860 and –0.763 along the ArKrXe series. The values of qB on the FNgBS molecules become relatively more positive, viz. 0.214, 0.010, and 0.055 on FArBS, FKrBS, and FXeBS, respectively. The qS value however remains almost the same as that on the minima. Although Mulliken population analysis provides a reasonable description of the bonding interactions, its basis set dependence is a well known fact. Therefore to get a better picture we have also performed natural bond orbital (NBO) analysis as implemented in MOLPRO, where the bond formation in the molecule is analyzed from the viewpoint of local orbital interaction. The calculated NBO charges are given in the Table 3. Both from the NBO and Mulliken analysis, it has been found that the FNg bond is significantly ionic in nature with considerable amount of charge separation. Nevertheless, both Mulliken and NBO charges clearly indicate that the FNgBS species exists as an ionic configuration, F(NgBS)+. However, charges calculated by both Mulliken and NBO methods suggest that there is considerable covalent character in the Ng–B bond. Apart from the charge redistribution analysis, we have calculated the dipole moments of all the predicted FNgBS molecules, which are useful for experimentalists in order to estimate environmental effects and spectral shifts. The dipole moment values for all the FNgBS molecules have been listed in Table S5 in the Supporting Information as obtained by using B3LYP and MP2 methods with B1 and B3 basis sets and employing CCSD(T) methods with B2 and B3 basis sets. 3.4. Analysis of Topological Properties of FNgBS Species The AIM model proposed by Bader55 is used here to study the bond critical point (BCP) properties. The MP2/B1 (B3LYP/B1) calculated AIM results of FNgBS species are given in Table 4, and for the purpose of comparison AIM properties for the FNgBO molecules calculated using the same levels are also reported in Table S6 (Supporting Information). The positive value of 2ρ suggest that F–Ng bond in both the series (FNgBS and FNgBO) has ionic character which decreases from Ar to Xe; whereas the Ng–B bond has considerable amount of covalent character due to the accumulation of electron density in the bond region with the negative values of 2ρ. Moreover, 2ρ 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

is found to be negative for the B–S bond in FNgBS while 2ρ > 0 for the B–O bond in FNgBO, which indicates towards more covalent nature of the B–S bond than the B–O bond. It is due to a greater charge/radius ratio, i.e., ionic potential of oxygen, which localizes electron around itself giving rise to ion–dipole type interaction. The calculated values of local energy density, Ed (r) (defined as: Ed (r) = G(r) + V(r), where G(r) and V(r) are the local kinetic energy and potential energy densities, respectively) are found to be negative for the FNg and NgB bonds, in both FNgBS and FNgBO molecules. A negative value of 2ρ and also a negative value of Ed for the Ng– B bond evidently indicate that this bond is associated with high covalent character. However, a positive value of 2ρ along with a negative value of Ed for the F–Ng bond implies that the nature of the bond is mainly ionic with small covalent contribution. Apart from the calculated AIM parameters at BCP, we have also plotted the electron density (ρ) and Laplacian of the electron density (2ρ) at various regions within the molecular plane in Figure 2 and Figure 3, respectively, for the FNgBS molecules. For the purpose of comparison we have included the corresponding plots for the FNgBO systems. The electron density contour plots of the FNgBS molecules are found to be almost identical to that of the FNgBO molecules, except in the BS and BO regions. In case of FNgBO molecules the BCP is located very close to the boron atom for the B-O bonds; however, the same is slightly away from the boron atom for the B-S bonds in FNgBS molecules. The contour lines corresponding to the (2ρ) distribution show more or less a uniform charge accumulation around the noble gas-boron-sulphur region in the FNgBS molecules, however, it is somewhat nonuniform in the case of FNgBO systems. Nevertheless, charge concentration in the Ng–B bonding region indicates that the Ng–B bond is rather covalent in nature. An in-depth analysis of Figure 3 reveals that the Ng–B bond becomes more covalent in going from Ar to Xe molecule in both FNgBS and FNgBO systems. From the Figure 3 it is also clear that the charge density is depleted around the bonding region of F–Ng bond in both FNgBS and FNgBO molecules, indicating that the F–Ng interaction is predominantly ionic in nature. Therefore, the ionic nature of FNg bond and the covalent character of NgB bond strongly suggest that these predicted molecules can be represented as F(NgBS)+. This conclusion has fair correspondence to the charge analysis results. Of late, Boggs and co-workers56 have performed an exhaustive study of nature of bonding involving noble gas compounds by considering G(r)/(r) at the BCP as an important parameter to assess the extent of covalency in a chemical bond. If Ed (r)

Noble-Gas-Inserted Fluoro(sulphido)boron (FNgBS, Ng = Ar, Kr, and

Recommend Documents