Effect of ionic charge on O H⋯Se hydrogen bond: A computational study

Accepted Manuscript Effect of ionic charge on O–H⋅ ⋅ ⋅Se hydrogen bond: A computational study Bijoya Das, Amrita Chakraborty, Shamik Chakraborty PII: DOI: Reference:

S2210-271X(16)30518-7 http://dx.doi.org/10.1016/j.comptc.2016.12.025 COMPTC 2342

To appear in:

Computational & Theoretical Chemistry

Received Date: Revised Date: Accepted Date:

19 April 2016 15 December 2016 15 December 2016

Please cite this article as: B. Das, A. Chakraborty, S. Chakraborty, Effect of ionic charge on O–H⋅ ⋅ ⋅Se hydrogen bond: A computational study, Computational & Theoretical Chemistry (2016), doi: http://dx.doi.org/10.1016/ j.comptc.2016.12.025

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Effect of ionic charge on O–H⋅⋅⋅Se hydrogen bond: A computational study 1

1

Bijoya Das, 2Amrita Chakraborty, and 1Shamik Chakraborty*

Department of Chemistry, Birla Institute of Technology and Science, Pilani Campus, Vidya Vihar, Pilani, Rajasthan - 333031, India. 2 Department of Chemistry, Techno India University, Sector V, Salt Lake City, Kolkata - 700091, West Bengal, India.

Abstract: Complexes between para-substituted cationic phenol and SeH2 have been investigated in electronic ground state at the B3LYP, B3PW91, and ωB97xD levels of theory using 6311++G(3df,3pd) basis set. Various electron- donating and withdrawing substituents (–NH2, –OH, –CH3, –H, –F, –Cl, –CN, and –NO2) are used to characterize electronic substituent effect on intermolecular +O–H⋅⋅⋅Se hydrogen bond. Electron withdrawing substituent increases hydrogen bond stabilization energy and red shift in O–H stretching frequency. Introduction of a positive charge transforms weak hydrogen bond of neutral O–H⋅⋅⋅Se type into a strong hydrogen bond. Complexation induced changes on various hydrogen bond parameters, such as, stabilization energy, change in O–H bond length, change in O–H stretching frequency, extent of charge transfer from hydrogen bond acceptor to donor, hydrogen bond orders, electron density at the hydrogen bond critical point exhibit + conventional electronic substitution effect. Stabilization energy of O–H⋅⋅⋅Y hydrogen bond are similar in the complexes between cationic phenol and SH2/SeH2, whereas it is almost twice with OH2 in case of +O–H⋅⋅⋅Y hydrogen bond. Key Words: Selenium, Hydrogen bond, Hammett parameter, DFT, Chalcogen, Phenol, Hydrogen selenide.

Corresponding Author: Shamik Chakraborty, E-mail: [email protected]

1. Introduction Among a conglomeration of non-covalent interactions those have been identified over the years, the hydrogen bonding (H-Bond) interactions are generally regarded as the most extensive and important. Investigation of H-Bond interaction of X–H⋅⋅⋅Y type in real and model systems is an important area of research due to its prevalent role in governing the properties of solvents, solute-solvent interactions, crystal structures, macromolecular properties, and various biological activities1-11. Commonly, X–H⋅⋅⋅Y type of hydrogen bonds are formed when X and Y are any one of the following electron rich elements, such as, O, N, F, Cl, Br, and I. Interaction energy of hydrogen bonds those involve electronegative elements falls in the regime of strong hydrogen bonding (>17 kJ mol-1) 7. The classical concept of electron rich atoms as hydrogen bond donor and acceptor in a X–H⋅⋅⋅Y arrangement has been expanded over the years. Less electronegative atoms, like, S, and C or even π-electron density of aromatic system have now been incorporated as hydrogen bond acceptor7, 12-17. There are several reports of weak H-Bond interactions (< 17 kJ mol-1), such as, C–H⋅⋅⋅O, C–H⋅⋅⋅N, C–H⋅⋅⋅π, X–H⋅⋅⋅S type. Out of the various weak hydrogen bonding interactions, the ability of a C-H group to serve as a proton donor has most extensively been investigated. The growing interest of C–H⋅⋅⋅X hydrogen bonding interaction has created a diverse literature database in the areas of chemistry, biochemistry, supramolecular chemistry, and biophysics18-24. Nowadays, active research is being conducted to recognize new hydrogen bond motifs. One of the main aims is to expand the available list of X and Y elements, i.e., hydrogen bond donor and acceptor, in the periodic table for a wider range of application. One of the fundamental properties that govern the hydrogen bond acceptor ability of Y element is the difference in electronegativity between H atom and Y centres. Sulphur atom, the second row element in chalcogen series has been reported to participate in hydrogen bonding interaction. A large number of experimental evidences in crystals, gas phase under isolated conditions, and theoretical prediction of the formation and stability of X–H⋅⋅⋅S hydrogen bond are available in literature13, 25-32. Electronegativity of sulphur, S, in Pauling’s scale is 2.5. The next element in chalcogen series is selenium having electronegativity of 2.4, which is slightly less than sulphur atom. Hence, a natural extension in the quest for new element in the list of hydrogen bond acceptor would certainly be selenium atom which is expected to form X–H⋅⋅⋅Se hydrogen bond based on electronegativity difference with the hydrogen atom33. Page 2 of 29

Selenium electrons are susceptible to excitation by light which in turn results in generation of an electric current. This has lead to wide use of selenium in photoelectric cells, light meters, rectifiers, and xerographic copying machines. Selenium chemistry began to attract wide acceptability only in recent years although it was discovered in 1817 and the first organoselenium compound, diethyl selenide, was prepared and isolated around 1869 34

. Organoselenium chemistry has been well-established as a field of research because of

the recent development of many new reactions leading to novel compounds with the potential for wide application in various disciplines34-38. For a long time, selenium was recognised as a toxic element. It has now been established that selenium is an essential micronutrients for human and animals. Selenium is biologically important element and present in mammals in the form of selenocysteine residue (Sec) which is considered as twenty-first amino acid. Selenocysteine residue is a common source of selenium in mammalian selenoproteins and its deficiency is associated with various fatal diseases including cancer, HIV, and cardiovascular disorder

39

. The field of selenium biochemistry

has expanded rapidly in last three decades. Many selenocysteine containing enzymes and proteins are reported and their specific functions in catalytic or physiological activities explain the requirement of trace amount of selenium for mammalian and bacterial survival4043

. Although, the organoselenium chemistry and selenium biochemistry have developed as

an independent field of research in last couple of decades but very little known about the role of selenium in selenium centred hydrogen bonding interaction (SeCHB) which could either be of X–H⋅⋅⋅Se or Se–H⋅⋅⋅X type of arrangement. SeCHB interaction might play important role towards the structure and function of selenoproteins. Herein, density functional theory (DFT) investigations of the X–H⋅⋅⋅Se hydrogen bond, with X = O atom, in a series of complexes between cationic para-substituted phenol +

([4-Q–C6H4–OH] ) and SeH2 have been carried out. One of the main aims is to understand the impact of positive charge on intermolecular +O–H⋅⋅⋅Se H-bond interaction. Moreover, the effect of electronic substitution on +O–H⋅⋅⋅Se H-bond has also been probed by changing substituent (–Q) at the para position of substituted phenol cation. The intramolecular X–H ⋅⋅⋅Se hydrogen bond interaction, with X = C, O, or N atoms, has been investigated in several organoselenium compounds which were characterized mainly by NMR, IR, X-ray crystallography, and quantum chemical calculations34,

44, 45

. The intramolecular O–H⋅⋅⋅Se

hydrogen bond was reported in 2-(methylseleno)ethanol while investigating conformational preference by vibrational spectroscopy and density functional theory calculations46. Page 3 of 29

Stabilisation energy of intramolecular O–H⋅⋅⋅Se hydrogen bond was estimated to be similar to that of O–H⋅⋅⋅O and O–H⋅⋅⋅S hydrogen bond interactions46. Moreover, structures and hydrogen bonding interaction energy of H2Se⋅⋅⋅HYH (with, Y = –O, –S, and –Se) dimers have been investigated at the DFT and MP2 level of theory. The interaction energy of these complexes are reported to be 13.02, 7.14, and 6.30 kJ mol-1 for Y = O, S, and Se atom, respectively, at the B3LYP level of calculation using 6-31++G(d,p) basis set. The binding energy of the cationic species is observed to be in reverse order along the same sequence of H-bond acceptor element, i.e. Y = O ( 56.28 kJ mol-1), S (60.48 kJ mole-1), and Se (69.3 kJ mol-1)47. Ionisation of the neutral H2Se ⋅⋅⋅HYH dimer leads to the migration of proton either from the Se atom or from Y centre. Recently, neutral intermolecular O–H⋅⋅⋅Se hydrogen bond has been investigated in the complexes of para-substituted phenol and SeH2. The typical stabilization energy of those complexes varies in the range of 4-15 kJ mol-1 depending on the nature of the substituent and method of calculation. Accordingly, intermolecular O–H⋅⋅⋅Se hydrogen bond has been classified as weak hydrogen bonding interaction7, 33. The interaction energy of intermolecular O–H⋅⋅⋅Se hydrogen bond is weaker compared to their classical analogues. For example, the computed binding energy for O–H ⋅⋅⋅O hydrogen bond in phenol–OH2 complex is in the range of 15-28 kJ mol-1 which is about three times higher compared to its higher analogue, i.e. O−H⋅⋅⋅Se33. Even O–H⋅⋅⋅S hydrogen bond interaction energy in phenol–SH2 complex is higher than that of O–H⋅⋅⋅Se in phenol– SeH2 complex. It is quite well known for classical hydrogen bond interaction that the introduction of a positive charge into the proton donor (X–H), or negative charge on the acceptor (Y), substantially enhance the hydrogen bond stabilization energy. Similar effect has also been observed in case of weak hydrogen bonding interaction of C–H⋅⋅⋅πtype48, C– H⋅⋅⋅O type 49. Herein, the effect of a positive charge on O–H ⋅⋅⋅Se H-bond interaction has been investigated to address whether charge enhanced interaction energy can transform the weak O–H⋅⋅⋅Se hydrogen bonding interaction into a strong +O–H⋅⋅⋅Se type. The present work represents the nature of intermolecular hydrogen bond interaction in the complexes between cationic para-substituted phenol and SeH2 complexes ([4-Q– C6H4–OH+⋅⋅⋅SeH2]), with Q = –NH2, –OH, –CH3, –H, –F, –Cl, –CN, and –NO2. Substituted phenols have been employed as the model system for the side chain of the naturally occurring amino acid tyrosine (Tyr). The SeH2 has been selected as the simplest model Page 4 of 29

system to characterize the hydrogen bond acceptor ability of Se atom and to characterize the effect of positive charge on possible Tyr-O–H⋅⋅⋅Se-Sec side chain hydrogen bond interaction in selenoproteins. As experimental and theoretical results are not available for cationic O–H⋅⋅⋅Se hydrogen bonding interaction of phenolic hydroxyl group, results reported for [4-Q-C6H4-OH+⋅⋅⋅SeH2] complexes provide a first impression of the effect of a positive charge in SeCHB interaction of O–H⋅⋅⋅Se type. Effect of electron density on +O–H⋅⋅⋅Se hydrogen bonding interaction has been investigated by changing the substituents (–Q) at the para position of cationic phenol. Complexation induced changes on various hydrogen bonding parameters, such as, stabilization energy, change in O–H bond length, shift in O–H stretching frequency, extent of electron density transfer from Se atom to cationic phenol moiety have been investigated. Moreover, natural resonance theory (NRT) based bond orders have been explored to determine the effect of electronic substitution effect on +O– H⋅⋅⋅Se type of hydrogen bonding interaction. The quantum theory of atoms-in-molecules (QTAIM) has also been used to investigate the neutral and cationic O–H⋅⋅⋅Se intermolecular hydrogen bonding interaction using electronic densities.

2. Computational Methods All the calculations are carried out using the Gaussian09 software package50. The density functional theory (DFT) calculation has been applied using B3LYP51-53, B3PW9154, and B97xD55 functionals in conjunction with the 6-311++G(3df,3pd) basis set56. The optimised geometry of the monomer and the hydrogen bonded complexes are obtained by relaxing all the coordinates. The identification of the local minimum is ensured by harmonic vibrational frequency analysis of each of the optimized structures. The hydrogen bond stabilisation energy of the [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes is calculated in accordance with equation 1, ,

The corrected energy term (∆



 

Equation 1

, , for the hydrogen bond interaction is the sum of the

uncorrected energy and all the corrections. Different terms used in the above equation are described as: E is the uncorrected energy of hydrogen bond formation determined as the difference between the fully optimized geometries of the complexes with the full complex

Page 5 of 29

   basis set (  ) and that of the monomers, A (  ) and B (  ), in respective monomer

basis set (Equation 2).  

 

 

Equation 2

∆  is the unscaled zero-point vibrational energy (ZPE) correction determined as, the difference between the zero point energy of the complex and that of the monomers. ∆   is the correction of the basis set superposition error (BSSE)57 in the counterpoise method as described by Boys and Bernardi58, Equation.  

 

 

 

 

Equation 3

In a complex A-B, for instance, the superposition of the basis sets of A and B leads to a much lower energy for AB as compared to the true energy. This leads to the higher binding energy. The counterpoise method is the most commonly used method for BSSE correction. In this method, the energy of the monomer constituents A and B are calculated separately   within the basis set of the whole complex A-B, i.e.  and  , respectively. Moreover,

the energy of the monomers are computed on their own basis set,

  and

  ,

respectively. ∆   is calculated in accordance to Equation 3. The charge transfer effect between the hydrogen bond donor and acceptor molecules are evaluated by the Natural Bond Orbital (NBO) analysis59-61. Electron occupancy in various orbitals, second order perturbation stabilisation energy ( E(2)), and Natural Resonance Theory (NRT) based bond orders are evaluated using NBO population analysis

62

. All the NBO and NRT analyses are carried out using NBO 6.0 software

63

package . The quantum theory of atoms-in-molecules (QTAIM) is used to explore the electron densities and to obtain further insight into the intermolecular hydrogen bonding interactions in [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes64-66. The topological parameters of electron densities of the X–H (X=O) bond as well as the H⋅⋅⋅Y (Y = Se) interactions are computed at the bond critical points (BCPs) using the AIM2000 program package67. The wavefunction obtained at the B3LYP, B3PW91, and B97xD level of theory using 6-311++G(3df,3pd) basis set have been used to calculate the electron density (r) and Laplacian of electron density   at the bond critical points. 3. Results and Discussion [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes may exist in two different configurations, [4-Q– C6H4–OH ⋅⋅⋅SeH2] and [4-Q–C6H4–OH⋅⋅⋅SeH2 ] which are connected by intracluster charge +

+

Page 6 of 29

transfer process. The preferred site of the positive charge depends on the relative ionisation potential (IP) of the 4-Q–C6H4–OH and the SeH2. As the ionisation potential of SeH2 (9.886±0.002/9.897±0.002 eV

68, 69

) is substantially higher compared to 4-H–C6H4–OH

(8.508±0.001 eV 70), cluster type of [4-H–C6H4–OH+⋅⋅⋅SeH2] is considered for the formation of [4-H–C6H4–OH⋅⋅⋅SeH2]+complexes. Ionisation potential of substituted phenols, 4-Q–C6H4– OH, is substantially less compared to SeH2. Thus,[4-Q–C6H4–OH ⋅⋅⋅SeH2] is considered for +

the formation of [4-Q–C6H4–OH⋅⋅⋅SeH2]+complexes as well. The adiabatic ionisation energy of all the substituted phenols and SeH2 have been calculated and are presented in Table S1 (Supporting Information) along with experimental values for an easy comparison. The experimental IP values are within 2-4% of that obtained at the B3LYP, B3PW91, and B97xD level of theory using 6-311++G(3df,3pd) basis set (Table S1, Supporting Information) which in turn justify validity of the adopted methodology for the investigation of such hydrogen bonded interaction. The ground state optimized geometry of [4-H–C6H4–OH ⋅⋅⋅SeH2] complex at the +

B3LYP/6-311++G(3df,3pd) level is presented in Figure 1. The geometrical parameters of the O-H⋅⋅⋅Se H-bond motif in various substituted [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes are very similar and summarized in Table 1. The hydrogen bond angle, ∠ O-H-Se, deviates from linearity by about 10° in [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes. The hydrogen bond angle remains unaltered by substituting the para-hydrogen of phenol either by electron donating or electron withdrawing substituents. The electronic effect of substitution is negligible on the hydrogen bond angle in [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes (Table 1). The O–H⋅⋅⋅Se hydrogen bond in [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes is characterized by computed distance between the oxygen atom and selenium atom (RO−Se) at the B3LYP (B3PW91/ B97xD) B97xD level.

RO−Se distance is in the range of 3.3604 Å (3.3242/3.3420 Å) to 3.2614 Å (3.2264/3.2370 Å) in these complexes. The distance between the hydrogen centre and selenium centre, H⋅⋅⋅Se (RH−Se), is calculated to be in the range of 2.3795 to 2.2587 Å, 2.3412 to 2.2201 Å, and 2.3623 to 2.2350Å at the B3LYP, B3PW91, and B97xD levels of theory, respectively.

RO−Se and RH−Se distances exhibit a gradual decrease with an enhancement in the electronic withdrawing ability of the substituents except for Q = –F and –Cl in all the three computational levels. Gradual decrease in the intermolecular distance, RO−Se, and hydrogen bond length RH−Se in [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes with increasing electron withdrawing

Page 7 of 29

ability of the substituents indicate the formation of stronger hydrogen bond with electron +

withdrawing substituents at the para position of [4-Q–C6H4–OH ]. The interaction energy of ionic O–H⋅⋅⋅Se hydrogen bond in [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes is calculated using unscaled zero point vibrational energy along with BSSE corrections ( EHB,CP) and are presented in Table 2. BSSE uncorrected stabilisation energy ( EHB) have also been reported in the same table to estimate the effect of BSSE correction at the level of calculations using 6-311++G(3df,3pd) basis set for selenium centred hydrogen bonding interaction. Magnitude of BSSE correction is quite small and it is found to be within three percent of the corrected binding energy. EHB and EHB,CP are always higher at the dispersion corrected B97xD level of calculation compared to that obtained at the B3LYP and B3PW91 levels. The typical hydrogen bond stabilization energy EHB,CP varies in the range of 32-54 kJ mol-1 depending on the nature of the substituents (–Q) and method of calculations. Hence, the ionic O–H⋅⋅⋅Se hydrogen bond interaction falls in the regime of strong H-bond interaction compared to its neutral analogue which is weak H-bond type

7, 33

.

An electron withdrawing substituent enhances H-bond interaction energy as the acidity of the phenolic proton gets increased whereas an electron donating substituent does in opposition. Hammett drew attention to the fact that all the substituents located either at meta (m) or para (p) positions in the benzene ring are exerting a similar effect in a variety of dissimilar reactions. Quantitatively, the effect of a substituent, relative to that of hydrogen is obtained by Hammett equation (Equation 4) 71-74 Substituent effect = log 

೉

ಹ



Equation 4

which correlates dissociation constants of substituted benzoic acid (KX) to that of the parent compound, benzoic acid (KH). In this relation,  , is known as the substituent constant. The magnitude of  depends on the relative location of the substituent in the benzene ring, i.e.,  ,  . Electron-withdrawing and electron-donating substituents are characterized by negative and positive values of σ, respectively. The reference point is hydrogen which has σ = 0.0. The slope of a Hammett plot of log 

೉

ಹ

against  is known as reaction constant ( ).

This is a measure of the sensitivity of a reaction to the effect of electronic perturbation. It is evident from Equation 4 that the value of

= 1.00 for the dissociation of benzoic acid in

water at 25° C. Thus, the reaction of dissociation of benzoic acid is considered to be the standard to compare susceptibilities or electronic demands of any other reaction. A positive

Page 8 of 29

value indicates an enhancement in the rate of the reaction induced by electronic perturbation due to electron withdrawal. Additionally, a negative

value indicates an

enhancement in the rate of the reaction induced by electronic perturbation due to electron donation. The magnitude of

provides an idea to measure the degree to which a reaction

responds to substituents. The hydrogen bond stabilisation energy of the [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes is plotted against the Hammett substituent constants ( p)71-74 and is presented in Figure 2. The correlation coefficient (R) of the regression between the interaction energy ( EHB,CP) and

p

is

0.9164 (Figure 2a), 0.9226 (Figure 2b), and 0.9429 (Figure 2c) at the B3LYP, B3PW91, and B97xD level of theories, respectively. The corresponding slopes are -9.44, -10.10 and 12.20 kJ mol-1. Therefore, cationic O–H⋅⋅⋅Se hydrogen bond follows the conventional electronic substituent effects (Figure 2) as observed in case of neutral O–H ⋅⋅⋅ Se hydrogen bond33. One of the most important observable parameters for the intermolecular hydrogen bond formation of X–H⋅⋅⋅Y type is the complexation induced change in the X–H bond length (RO−H) and subsequent change in the X–H stretching frequency (νO−H) of the hydrogen bond donor molecule in the complex compared to the isolated state. The hydrogen bond induced alteration in the O–H bond length ( RO–H) of the hydrogen bond donor molecule [4−Q−C6H4−OH+] in [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes are reported in Table 2 along with the corresponding shift in the O–H stretching frequency ( νO–H). An elongation in the O–H bond length of the substituted phenol is observed in the [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes compared to the isolated molecules. These structural changes directly translate into the vibrational spectral properties which exhibits substantial red shift in the O−H stretching frequency of [4-Q–C6H4–OH+] upon complex formation. The complexation induced O–H bond length elongation is maximum at the B3PW91 level of calculations (0.0277 – 0.0458 Å) compared to that obtained at the B3LYP level (0.0246 – 0.0411 Å), and B97xD level (0.0230 – 0.0400 Å). Similar trend has been observed in case of complexation induced shift in the O−H stretching frequency of [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes. The red shift in O−H stretching frequency is in the range of 574-906 cm-1 at the B3PW91 level followed by 515825 cm-1 at the B3LYP level, and 483-813 cm-1 at the B97xD level of theory. The extent of elongation in the O–H bond length and subsequent red shift in the O–H stretching frequency show nice correlation with the substituent

Page 9 of 29

p

constants. The correlation coefficient

(R) between the change in O–H bond length ( RO−H) and Hammett

p

constant is 0.9197,

0.9233, and 0.9413 at the B3LYP, B3PW91, and B97xD level of theory, respectively (Figure S1, Supporting Information). Similarly, the correlation coefficient (R) between the change in the O–H stretching frequency ( νO−H) and substituent

p

constants is 0.9227,

0.0.9249, and 0.0.9418 at the B3LYP, B3PW91, and B97xD level of theory, respectively (Figure S2, Supporting Information). The regression slopes are 0.0101 Å, 0.0111Å, and 0.0103 Å for O–H bond length change and –190 cm-1, –204 cm-1, and –201 cm-1 for change in O–H stretching frequency at the B3LYP, B3PW91, and

B97xD level of theory,

respectively. Consequently, it can be concluded that an electron withdrawing substituent brings about more elongation in O–H bond length and subsequent red shift in O−H stretching frequency compared to an electron donating substituent. Similar trend has been observed for hydrogen bond stabilisation energy of these complexes. The correlation between hydrogen bond stabilisation energy with change in O–H bond length and O–H stretching frequency is excellent. The correlation plot of hydrogen bond stabilisation energy with change in O–H bond length is presented in Figure 3. The correlation coefficient (R) of the regression between EHB,CP and RO−H are 0.9972, 0.9956, and 0.9849 at the B3LYP, B3PW91, and

B97xD level of theory, respectively. The change in O–H stretching

frequency varies linearly with the change in O–H bond length. The correlation coefficient (R) of the regression between νO−H and RO−H is found to be 0.9998 at the B3LYP level, 0.9996 at the B3PW91 level, and 0.9997 at the B97xD level of theory and are presented in Figure 4. Thus, the cationic O–H⋅⋅⋅Se hydrogen bond in [4-Q–C6H4–OH+⋅⋅⋅SeH2] complexes follows the conventional mechanism of H-bond formation. The NBO population analysis is a reliable approach to delineate the formation of hydrogen bond. Moreover, NBO analysis elucidates the variation in hydrogen bond parameters including stabilisation energy in concordance with the basic concepts of chemical bonding62. Precise information is obtained about the electron density shift due to hydrogen bond formation at the hydrogen bond acceptor centre (Y) and in the hydrogen bond donor group (O–H), especially in the anti-bonding orbital of X−H (σ*O-H). In general, the NBO analysis converts the delocalized molecular orbitals into localised ones. The interaction between the occupied orbital, like, lone pair on Y and anti-bonding orbital of X−H bond represents the deviation of the molecule from the Lewis structure and can be considered as a measure of the delocalization due to the presence of hydrogen bonding interaction. All these descriptors provide sensitive information to investigate hydrogen bond

Page 10 of 29

interaction at atomic level. The acceptor NBO (Figure 5a), the donor NBO (Figure 5b), and σ*O-H, (Figure 5c) are depicted in Figure 5. The the donor-acceptor interacting NBOs, LPSe→ stabilisation energies, ΔEij(2), for the charge transfer from Se atom (Y), i.e. lone pair of electron (LP of Se), in SeH2 to the *O−H of [4-Q−C6H4−OH]+ is obtained by second order perturbation theory analysis of the Fock Matrix in the NBO calculation of [4Q−C6H4−OH+⋅⋅⋅SeH2] complexes which are presented in Table 3. Electron occupancy in each of these orbitals is presented in Table 3 as well for easy comparison. Electron occupancy in *O−H orbital of [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes varies in the range of 0.0770 e to 0.1314 e depending on the substituents and the level of calculation. Correspondingly, lone pair occupancy on Se atom has decreased in the range of 1.9216 e to 1.8655 e. It is evident that there is a significant intermolecular charge transfer from the lone pairs of Se (Φi) to the *O−H orbital (Φj) in [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes due to the formation of cationic O−H⋅⋅⋅Se hydrogen bond. The intermolecular charge transfer interaction from the donor orbital (Φi of Se) to the acceptor orbital, *O−H, is the dominant contributor to stabilise +

O−H⋅⋅⋅Se hydrogen bond as all other stabilisation energy involving any other orbitals is

negligible compared to this interaction. Extent of electron density transfer and associated second order interaction energy, Eij(2), enhances steadily from electron donating group, Q = −NH2, to electron withdrawing group, Q = −CN. Stabilization of cationic O−H⋅⋅⋅Se hydrogen bonding interaction enhances with an increase in electron withdrawing ability of the substituents. Extent of charge transfer in [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes increases from 0.0799 to 0.1201 at the B3LYP level, 0.0877 to 0.1314 at the B3PW91 level, and 0.0770 to 0.1174 at the

B97xD level of theory. Similarly, the enhancement in second order

interaction energy, Eij(2), is found to be in the range of 17.57 to 27.18 kJ mol-1, 20.04 to 31.15 kJ mol-1, and 24.15 to 38.16 kJ mol-1 at the B3LYP, B3PW91, and B97xD level of theory, respectively. Electron occupancy in *O−H orbital of [4-Q−C6H4−OH+] in the complexes correlates nicely with the second order interaction energy, Eij(2), and hydrogen bond parameters, such as, νO−H and RO−H. NRT bond order

4, 62, 75, 76

of the O−H bond (bOH) is calculated in the range of 0.9545-

0.9259. The missing bOH valency of the cationic O−H⋅⋅⋅Se hydrogen bond motif is distributed into the hydrogen bond component (bH⋅⋅⋅Se) and a long range bond component between the oxygen and selenium atoms (bSe⋅⋅⋅O). The bH⋅⋅⋅Se and bSe⋅⋅⋅O bond orders for [4Q−C6H4−OH+⋅⋅⋅SeH2] complexes are presented in Table 3 which correlate well with the Page 11 of 29

p

constants. Thus, these complexes get stabilized by three centre four electron (3c/4e) resonance type interaction33. The total bond order associated with the +O−H⋅⋅⋅Se hydrogen bond motif (btotal = bH⋅⋅⋅Se + bSe⋅⋅⋅O, Table 3) shows better correlation with

p

constants. The

correlation (R) for the regression between btotal and complexation induced shift in O−H stretching frequency is 0.9991, 0.9993, and 0.9996 at the B3LYP, B3PW91, and B97xD level of theory, respectively (Figure 6). NRT bond orders have good correlation with other hydrogen bond parameters, such as, hydrogen bond stabilisation energy and complexation induced change in O−H bond length. Mapping of topological properties of the electron density to Lewis structure representation of molecules is provided by QTAIM analysis64. QTAIM has been used to characterise hydrogen bonding interaction of different strengths in a variety of molecular complexes. Existence of X−H···Y hydrogen bond is characterised by a bond path connecting H and Y atoms with a (3, -1) bond critical point. The electron density at the BCP,

H···Se

, and

the Laplacian of electron density at the BCP,   , depends on the level of calculations. Still, a well defined range has been proposed to confirm the existence of hydrogen bonds in terms of electron density at the BCP and the corresponding Laplacian. The values of

H···Se



and   are expected to be within the range of 0.002 – 0.04 a.u. and 0.02 – 0.15 a.u., respectively. The nature of interaction can be predicted from relative values of of the Laplacian. Large

H···Se

H···Se

and sign

values along with negative Laplacian values are indicator for

shared interaction that is characteristic of covalent bonds. However, small

H···Se

values in

combination with positive Laplacian values are indicator of closed-shell interactions typically observed in ionic bonds, hydrogen bonds, and van der Waals’ interactions. Herein, the +

O−H⋅⋅⋅Se hydrogen bond is characterized based on the criteria of magnitude of

H···Se

and

sign of Laplacian at BCP in [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes. The bond paths are presented in Figure7 along with all the BCPs for the hydrogen bonded complexes, [4Q−C6H4−OH+⋅⋅⋅SeH2]. The values of

H···Se

for these complexes at various level of calculations

are obtained in the range of 0.026 – 0.037 a.u. These values are well within the specified range for the formation of hydrogen bond in terms of electron density at the bond critical point. Therefore, the formation of +O−H⋅⋅⋅Se hydrogen bond in [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes is well justified from QTAIM. The Laplacian,   , of charge densities corresponding to these complexes at the BCP are computed in the range of 0.008–0.009

Page 12 of 29

a.u. These values are less than the predicted values for hydrogen bonding interaction whereas the sign of Laplacian confirm the existence of hydrogen bonding interaction. The electron density at the H⋅⋅⋅Se BCP is a meaningful descriptor for hydrogen bonding interaction which is expected to follow the trends observed for traditional parameters of hydrogen bonding. A steady increase in the

H···Se

value has been observed

irrespective of the method of calculations with the increase in the electron withdrawing ability of the substituents (–Q) except for Q = –F and –Cl, those show a slight deviation. The correlation coefficient (R) of the regression between the Hammett Se

p

constants and

H···

(ρHB) are 0.9143, 0.9181, and 0.9310 at the B3LYP, B3PW91, and B97xD level of theory,

respectively (Figure S3, Supporting Information). Furthermore, the electron density at the bond critical point shows linear correlation with all the traditional hydrogen bond parameters, such as, binding energy, change in O−H stretching frequency (Figure 8) and bond length upon complexation. Summary of these results are presented in Table 4. Bader theory of atoms in molecules provides additional parameters to characterise hydrogen bonding interaction. The characteristics of H⋅⋅⋅Se BCP for the ionic complexes are already discussed in the previous section. Electron density at the BCP of +O–H⋅⋅⋅Se hydrogen bond in [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes are always two to three times higher compared to that in the neutral complexes, [4-Q−C6H4−OH⋅⋅⋅SeH2]. Similarly, the Laplacian of the electronic charge density at O−H···Se hydrogen bond critical point is always higher in the cationic case compared to its neutral analogue (Table 4). However, O−H bond critical point properties are also consequential to explain the cationic +O−H⋅⋅⋅Se hydrogen bond in [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes compared to neutral O−H⋅⋅⋅Se hydrogen bond in [4Q−C6H4−OH⋅⋅⋅SeH2] complexes. The covalent O−H bond has been observed to elongate as a result of hydrogen bond formation. Electron density at O−H BCP in the complexes has been reduced by 0.03 – 0.04 a.u. compared to the monomer. Similarly, there is an increase in the Laplacian of electron density at the bond critical point. As expected, electron density and Laplacian of electron density at O−H BCP are substantially higher in neutral complexes (Table 4). As discussed above, the neutral intermolecular O−H···Se hydrogen bond stabilization energy in [4-Q−C6H4−OH⋅⋅⋅SeH2] complexes is reported in the range of 4-15 kJ mol-133 which falls in the regime of weak hydrogen bonding interaction. Addition of a positive charge by removing one electron from the proton donor molecule significantly enhances the strength

Page 13 of 29

of +O−H···Se hydrogen bond stabilization energy in [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes which is in the range of 32-54 kJ mol-1and falls in the regime of strong hydrogen bonding interaction33. This four to five times magnification of stabilization energy in +O−H···Se hydrogen bond compared to neutral O−H···Se analogues is expected to be due to more positive bridging proton that can better interact with the Se atom on the proton acceptor (Table S-2, Supporting Information). Magnification in hydrogen bond stabilization energy due to the introduction of positive charge is more significant in case of electron donating groups compared to electron withdrawing groups (Table 2 and Reference 33). Nature of the substituent has direct impact on the ionisation potential of substituted phenols, i.e., ionization potential increases with electron withdrawing ability of the substituent at para position. The partial positive charge on the bridging proton is slightly higher (≥ 0.02 e) in case of electron donating groups in cationic complexes compared to the neutral one. There is almost no difference in the partial positive charge of the bridging proton in case of phenol and other substituted phenols with electron withdrawing groups (Q = −F, −Cl, −CN, −NO2). Simple model can be used to calculate the electrostatic component of the H-bond stabilization by considering the partial positive charge on bridging proton, selenium atom, and the internuclear distance. There is almost no difference in the calculated values of electrostatic components using the simple model for cationic and neutral O−H···Se hydrogen bond. The presence of aromatic ring in the proton donor unit must have key role to stabilize the three centre four electron (3c/4e) bonded complexes in the presence of a positive charge compared to the neutral complex. It has been reported that neutral O−H · · ·Se hydrogen bond in [4-Q−C6H4−OH ⋅⋅⋅SeH2] complexes is stabilized partly by the resonance assisted hydrogen bond (RAHB)77,

78

coupling of SeH2 unit to the phenol.

Similarly, RAHB coupling between the SeH2 unit and substituted phenols have been confirmed in [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes which is evident from the shortening of C−O bond length of the ionic phenol in the complexes compared to the monomer (Table S3, Supporting Information). The change in the C−O bond length in the ionic complex is significantly higher than that in neutral complexes. Hence, RAHB coupling plays important role to stabilize +O−H · · ·Se hydrogen bond. This is further justified by the observation that the significant portion of the positive charge is redistributed in the aromatic ring (−C6H4−) due to resonance (Table S-2, Supporting Information). The analogue of neutral and cationic O−H···Se hydrogen bond has been investigated in the simple system of

Page 14 of 29

H2Se···HOH. Proton migration has been reported in the ionic state which leads to either (H2SeH···OH)+ with binding energy of 56.28 kJ mol-1or (HSe· · ·HOH2)+ with binding energy of 107.01 kJ mol-1 at the B3LYP level of calculation using 6-31++G(d,p) basis set. No such proton migration has been observed in [4−Q−C6H4−OH⋅⋅⋅SeH2]+ complexes. The presence of positive charge in the proton donor unit has significant impact on geometrical parameters of O−H···Se hydrogen bond. Hydrogen bond angle, ∠ O-H-Se, in +

O−H · · ·Se hydrogen bond is more linear compared to the neutral O−H · · ·Se. As observed

in case of binding energy the impact on hydrogen bond angle is more pronounced for electron donating groups compared to the electron withdrawing groups. The hydrogen bond distance, RH···Se, and the distance between O and Se atom, RO-Se, is much less (0.3 to 0. 4 Å) in the ionic complexes compared to their neutral analogues. Complexation induced change in O−H bond length (ΔRO−H) in case of +O−H···Se hydrogen bond is five to six times higher compared to that in neutral O−H···Se hydrogen bond. Magnification in lengthening of O-H bond length due to addition of a positive charge in the hydrogen bond donor system, [4Q−C6H4−OH], is translated into the corresponding red shift of the O−H stretching frequency (ΔνO-H). The computed red shift of νO-H is in the range of 480 to 900 cm-1 in [4Q−C6H4−OH+⋅⋅⋅SeH2]. The calculated red shift in the O−H stretching frequency of the neutral -1

complexes was in the range of 120 to 210 cm . The results of NBO population analysis very nicely elaborate the significant change in the hydrogen bond stabilization energy and all other hydrogen bond parameters in the O−H···Se hydrogen bond by addition of a positive charge. The extent of electron density migration from Se atom to the hydrogen bond donor system, [4-Q−C6H4−OH], is significantly higher, i.e. ≥ 0.07 e, in case of +O−H···Se hydrogen bond. Consequently, electron density accumulation in σ*O−H has increased by 0.05 to 0.08 e due to the presence of a positive 

charge in O−H···Se hydrogen bonded complexes. The stabilization energy, ∆ , for the electron density transfer from Se atom to the σ*O−H exhibits four to five times enhancement due to the presence of the positive charge. NRT bond order (btotal) has also been increased by almost the same extent in [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes compared to [4Q−C6H4−OH⋅⋅⋅SeH2] complexes. Topological properties of the neutral complexes that have been presented in Table 4 have not been reported before to the best of the author's knowledge. The bond paths in [4-

Page 15 of 29

Q−C6H4−OH⋅⋅⋅SeH2] complexes are very similar to that of the cationic complex (Figure 7). However, the calculated electron density at the hydrogen bond critical point (ρH···Se) is in the range of 0.0123 to 0.0161 a.u. (Table 4) which is slightly lower than the accepted range for hydrogen bonding interaction. This implies the weak nature of the neutral O−H···Se hydrogen bond and indicates that charge is mostly localized in the chemical bonds. Electron density at the O−H bond critical point of [4-Q−C6H4−OH] is close to 0.37 a.u. which has reduced to 0.33 au in case of [4-Q−C6H4−OH]+. This is reflected in the significant increase in calculated ρH···Se in +O−H···Se hydrogen bond which is more than twice compared to the neutral case. The [4-Q−C6H4−OH+⋅⋅⋅YH2] type of complexes, with Y = O and S atoms, have also been investigated at the same level of theory, i.e., at the B3LYP, B3PW91, B97xD levels using 6-311++G(3df,3pd) basis set. An easy comparison of H-bond acceptor ability of Se atom compared to other members in chalcogen series, i.e., O and S atoms has been carried out to understand the role of these hydrogen bonds in various macromolecular systems including bio-molecules. Salient H-bond parameters, such as, binding energy, complexation induced change in O−H bond length (ΔRO−H) and stretching frequencies (ΔνO−H) have been calculated and are presented in Table 5. The calculated binding energy of +O−H · · · O H-bond interaction in [4-Q−C6H4−OH+⋅⋅⋅OH2] complex is in the range of 71.21 to 75.73 kJ mol-1 which is in close agreement with the experimental dissociation energy of 73.33 kJ mol-1 in the gas phase79. H-bond interaction energy for +O−H···S type in [4-Q−C6H4−OH ⋅⋅⋅SH2]+ complex is calculated as 44.51 kJ mol-1, 45.15 kJ mol-1, 48.13 kJ mol-1 at the B3LYP, B3PW91, and B97xD levels of calculation using 6-311++G(3df,3pd) basis set, respectively. The H-bond interaction energy in [4-Q−C6H4−OH+⋅⋅⋅YH2] complexes is observed to increase linearly with an enhancement in electronegativity difference of H and Y atoms, Figure 9. The correlation coefficients (R) of the regression between the H-bond interaction energy and electronegativity difference is 0.9973, 0.9974, and 0.9971 at the B3LYP, B3PW91, and B97xD levels of calculation. Similar trends have also been observed for hydrogen bond induced changes in O−H bond length (ΔRO−H) and O−H stretching frequency (ΔνO−H). The correlation coefficients (R) of the regression between the ΔνO−H and electronegativity difference is 0.9959, 0.9997, and 0.9989 at the B3LYP, B3PW91, and B97xD levels of calculation, Figure 10. 5. Conclusion

Page 16 of 29

The present work reports the first investigation and characterization of +O−H···Se type of hydrogen bonding interaction in [4-Q−C6H4−OH⋅⋅⋅SeH2]+ complexes, with Q = –NH2, –OH, –CH3, –H, –F, –Cl, –CN, and –NO2 substituents. These complexes are formed by +

O−H···Se H-bond interaction which gets stabilized by three centre four electron (3c/4e)

resonance type interaction62, 75. The interaction energy of +O−H···Se H-bond interaction is in the range of 32-57 kJ mol-1 which falls in the regime of the strong hydrogen bonding interaction. Weak O−H···Se hydrogen bonding interaction is converted into strong hydrogen bond type by addition of a positive charge in the hydrogen bond donor molecule. An electron withdrawing para substituent increases the interaction energy for these hydrogen bonds. The magnification of the hydrogen bond stabilization energy in [4-Q−C6H4−OH ⋅⋅⋅SeH2]+ complexes due to the presence of a positive charge compared to their neutral analogues is mostly governed by resonance assisted hydrogen bond coupling of SeH2 unit to the [4-Q−C6H4−OH]+ and not by electrostatics. The O−H stretching frequency in intermolecular +O−H···Se hydrogen bond is red shifted by 574 to 906 cm-1 along with an elongation in the O−H bond length. The magnitude of red shift in O−H stretching frequency and elongation in O−H bond length increases with an increase in the electron withdrawing ability of the substituent (−Q) at the para position of [4-Q−C6H4−OH]+. Primarily, the +

O−H···Se H-bond is stabilized by transfer of electron density from one of the lone pairs on

Se atom to the anti-bonding orbital of O−H. Extent of electron density transfer from Se atom to [4-Q−C6H4−OH]+ also increases by an electron withdrawing substituent. Topological properties of the electron density is obtained using QTAIM to characterize both +O−H···Se and O−H···Se hydrogen bond. Stronger H-bond interaction is justifies from the electron density values at the hydrogen bond critical point. Various properties of the intermolecular +

O−H···Se hydrogen bond in [4-Q−C6H4−OH⋅⋅⋅SeH2]+ complexes exhibits conventional

electronic substituent effect. H-bond acceptor ability of first three members in chalcogen series, O-, S-, and Se- atom, has been investigated in [4-Q−C6H4−OH⋅⋅⋅YH2]+ complexes, with Y = O, S, and Se. Important parameters for intermolecular hydrogen bond formation, such as, hydrogen bond interaction energy, change in O-H bond length and stretching frequency, are observed to vary linearly among the first three members of chalcogen series with their electronegativity difference with H atom. 6. Acknowledgement

Page 17 of 29

This study is supported by the financial support received from Science and Engineering Research Board, Government of India [SB/FT/CS-106/2013].

Page 18 of 29

Figure Caption Figure 1. Optimized geometry of cationic phenol-SeH2 complex [4 H-C6H4 OH+⋅⋅⋅SeH2] obtained at the B3LYP level of calculation using 6-311++G(3df,3pd) basis set. Figure 2. Correlation plots between hydrogen bond stabilization energy ( EHB,CP in kJ mol-1)

vs. Hammett substituent σp constant of [4 Q-C6H4 OH+⋅⋅⋅SeH2] complexes.

EHB,CP is

calculated at the (a) B3LYP, (b) B3PW91, and (c) B97xD level of theory using 6311++G(3df,3pd) basis set.

Figure 3. Correlation plots between complexation induced change in the O•H H bond length (ΔRO•HH, in Å) of [4•Q Q-C6H4•OH+⋅⋅⋅SeH2] complexes vs. hydrogen bond stabilization energy ( EHB,CP in kJ mol-1) obtained at the (a) B3LYP, (b) B3PW91, and (c) B97xD level of theory using 6-311++G(3df,3pd) basis set.

Figure 4. Correlation plots between complexation induced change in the O•H H bond length (ΔRO•HH, in Å) vs. complexation induced change in the O•H H stretching frequency ((ΔνO•HH) of [4•Q QC6H4•OH+⋅⋅⋅SeH2] complexes at the (a) B3LYP, (b) B3PW91, and (c) B97xD level of theory using 6-311++G(3df,3pd) basis set. Figure 5. The donor-acceptor interacting NBOs (npSe→ σ*O•H) H) involve in O•H···Se H···Se hydrogen bond of the [4•H H-C6H4•OH+⋅⋅⋅SeH2] complex obtained at the B3LYP level of calculation using 6-311++G(3df,3pd) basis set; (a) acceptor NBO (σ*O•H), H), (b) donor NBO (npSe), and (c) donor-acceptor interacting NBOs (npSe → σ*O•H) H).

Figure 6. Correlation plots between the natural resonance theory (NRT) bond order (btotal)

vs. complexation induced change in the O•H H stretching frequency (ΔνO•HH)of [4•Q QC6H4•OH+⋅⋅⋅SeH2] complexes obtained at the (a) B3LYP, (b) B3PW91, and (c) B97xD level of theory using 6-311++G(3df,3pd) basis set. Figure 7. The molecular graph of [4•H H-C6H4•OH+⋅⋅⋅SeH2] complex using wavefunction obtained at the B3LYP level of calculation with 6-311++G(3df,3pd) basis set.

Page 19 of 29

Figure 8. Correlation plots between the complexation induced change in the O•H H stretching Q-C6H4•OH+⋅⋅⋅SeH2] complexes vs. the electron density (ρHB) at the frequency (ΔνO•HH) of [4•Q H⋅⋅⋅Se bond critical point (BCP). ρHB is calculated using the wavefunction obtained at the (a) B3LYP, (b) B3PW91, and (c) B97xD level of theory using 6-311++G(3df,3pd) basis set.

Figure 9. Correlation plots between the hydrogen bond interaction energy (ΔEHB,CP) of [4H−C6H4−OH+⋅⋅⋅YH2] complexes (Y = O, S, and Se) vs. electronegativity difference between H and Y atoms (in Pauling scale). ΔEHB,CP is calculated at the (a) B3LYP, (b) B3PW91, and (c) B97xD level of theory using 6-311++G(3df,3pd) basis set.

Figure 10. Correlation plots between the complexation induced change in the O•H H stretching frequency (ΔνO•HH)of [4-H−C6H4−OH⋅⋅⋅YH2]+ complexes (Y = O, S, and Se) vs. electronegativity difference between H and Y atoms (in Pauling scale) obtained at the (a) B3LYP, (b) B3PW91, and (c) B97xD level of theory using 6-311++G(3df,3pd) basis set.

Page 20 of 29

Table 1 O−Se (RO−Se) and H−Se (RH−Se) bond lengths (Å) and hydrogen bond angle, ∠ O−H−Se, of the [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes obtained at B3LYP, B3PW91, and B97xD level of theory using 6-311++G(3df,3pd) basis set. Substituent (-Q)

Method

RO−Se

RH−Se

∠ O−H−Se

−NH2

B3LYP B3PW91 B97xD

3.3604 3.3242 3.3420

2.3795 2.3412 2.3623

169.9 170.1 172.5

−OH

B3LYP B3PW91 B97xD

3.3231 3.2871 3.3069

2.3350 2.2964 2.3209

170.2 170.4 172.6

−CH3

B3LYP B3PW91 B97xD

3.3100 3.2738 3.2920

2.3185 2.2794 2.3030

170.3 170.5 172.4

−H

B3LYP B3PW91 B97xD

3.2835 3.2468 3.2646

2.2864 2.2465 2.2709

170.1 170.2 172.1

−F

B3LYP B3PW91 B97xD

3.2886 3.2529 3.2721

2.2918 2.2531 2.2788

170.7 170.9 172.8

−Cl

B3LYP B3PW91 B97xD

3.3013 3.2647 3.2850

2.3086 2.2688 2.2941

170.2 170.5 172.8

−CN

B3LYP B3PW91 B97xD

3.2723 3.2361 3.2538

2.2725 2.2323 2.2559

170.3 170.7 172.9

−NO2

B3LYP B3PW91 B97xD

3.2614 3.2264 3.2370

2.2587 2.2201 2.2350

170.4 170.6 172.9

Page 21 of 29

Table 2 O−H bond length (Å), O−H stretching frequency (cm-1), and hydrogen bond energy (kJ mol-1) of the [4-Q−C6H4−OH+⋅⋅⋅ SeH2] complexes obtained at the B3LYP, B3PW91, and B97xD level of theory using 6-311++G(3df,3pd) basis set. -Q

Method

a

RO−H, m

b

RO−H, c

ΔRO−H

νO−H, m

c

d

νO−H, c

ΔνO−H

e

ΔEHB

ΔEHB,CP

−NH2

B3LYP 0.9671 0.9917 0.0246 3767 3252 -515 -33.03 -32.10 B3PW91 0.9657 0.9934 0.0277 3793 3219 -574 -33.23 -32.15 B97xD 0.9627 0.9857 0.0230 3844 3361 -483 -37.47 -36.36

−OH

B3LYP 0.9685 0.9984 0.0299 3750 3133 -617 -38.67 -37.70 B3PW91 0.9671 1.0005 0.0334 3776 3093 -683 -39.12 -37.99 B97XD 0.9641 0.9917 0.0276 3828 3250 -578 -42.83 -41.67

−CH3

B3LYP 0.9696 1.0016 0.0320 3734 3079 -655 -40.06 -39.10 B3PW91 0.9682 1.0040 0.0358 3760 3035 -725 -40.66 -39.53 B97XD 0.9653 0.9950 0.0297 3810 3192 -618 -44.20 -43.03

−H

B3LYP 0.9708 1.0075 0.0367 3719 2979 -740 -44.57 -43.54 B3PW91 0.9694 1.0105 0.0411 3744 2927 -817 -45.38 -44.19 B97XD 0.9664 1.0003 0.0339 3795 3095 -700 -48.47 -47.21

−F

B3LYP 0.9701 1.0060 0.0359 3729 3000 -729 -44.44 -43.44 B3PW91 0.9687 1.0087 0.0400 3755 2953 -802 -45.04 -43.87 B97XD 0.9656 0.9988 0.0332 3806 3122 -684 -48.22 -47.02

−Cl

B3LYP 0.9695 1.0028 0.0333 3736 3054 -682 -41.91 -40.93 B3PW91 0.9681 1.0055 0.0374 3762 3006 -756 -42.53 -41.38 B97XD 0.9652 0.9963 0.0311 3811 3164 -647 -46.62 -45.43

−CN

B3LYP 0.9707 1.0097 0.0390 3721 2934 -787 -47.01 -46.02 B3PW91 0.9693 1.0131 0.0438 3745 2877 -868 -47.99 -46.84 B97xD 0.9665 1.0033 0.0368 3794 3038 -756 -52.02 -50.81

−NO2

B3LYP 0.9714 1.0125 0.0411 3712 2887 -825 -48.47 -47.43 B3PW91 0.9699 1.0157 0.0458 3738 2832 -906 -49.26 -48.05 B97XD 0.9672 1.0072 0.0400 3784 2971 -813 -55.57 -54.31

a

O−H bond length of monomer; b O−H bond length of complex; c O−H stretching frequency of monomer; d O−H stretching frequency of monomer; e without BSSE correction

Page 22 of 29



Table 3 Stabilization energies ∆  for selected NBO pairs as obtained by second-order perturbation theory analysis of the Fock matrix in the NBO basis for [4-Q−C6H4−OH+⋅⋅⋅ SeH2] complexes. Occupation in each of the Φ i and Φ j orbitals are mentioned as well. Φ i designates the donor NBO and Φ the acceptor NBO. NRT bond orders, b , b , and b j H⋅⋅⋅Se Se⋅⋅⋅O total = bH⋅⋅⋅Se + bSe⋅⋅⋅O for O−H⋅⋅⋅Se H-bond unit. −Q

Method a

Occupancy, e (Φ i, LP2 Se)

Occupancy, e (Φ *(O−H)) j, σ

−NH2

B3LYP B3PW91 B97XD

1.9190 1.9111 1.9216

−OH

B3LYP B3PW91 B97XD

−CH3



btotal

0.0799 0.0877 0.0770

∆ (kcal mol-1) 17.57 20.04 24.15

0.0288 0.0112 0.0400 0.0327 0.0112 0.0439 0.0266 0.0116 0.0382

1.9030 1.8939 1.9066

0.0951 0.1040 0.0912

21.12 24.08 28.96

0.0365 0.0117 0.0482 0.0410 0.0119 0.0529 0.0348 0.0113 0.0461

B3LYP B3PW91 B97XD

1.8970 1.8872 1.9001

0.1012 0.1108 0.0979

22.56 25.76 31.19

0.0396 0.0117 0.0513 0.0454 0.0114 0.0568 0.0384 0.0113 0.0497

−H

B3LYP B3PW91 B97XD

1.8835 1.8725 1.8871

0.1142 0.1249 0.1103

25.65 29.34 35.57

0.0489 0.0100 0.0589 0.0543 0.0103 0.0646 0.0459 0.0105 0.0564

−F

B3LYP B3PW91 B97XD

1.8853 1.8751 1.8897

0. 1117 0.1218 0.1072

25.20 28.71 34.60

0.0445 0.0126 0.0571 0.0497 0.0132 0.0629 0.0420 0.0127 0.0547

−Cl

B3LYP B3PW91 B97XD

1.8928 1.8826 1.8959

0.1050 0.1150 0.1015

23.50 26.90 32.51

0.0410 0.0125 0.0535 0.0471 0.0120 0.0591 0.0397 0.0118 0.0515

B3LYP 1.8771 B3PW91 1.8655 B97XD 1.8793 a 6-311++G(3df,3pd) basis set

0.1201 0.1314 0.1175

27.18 31.15 38.16

0.0505 0.0115 0.0620 0.0564 0.0116 0.0680 0.0500 0.0109 0.0609

−CN

Page 23 of 29

bH⋅⋅⋅Se

bSe⋅⋅⋅O

Table 4 Values of electron densities and Laplacian,  , at the hydrogen bond critical point and O-H bond critical point for all the for all the [4-Q−C6H4−OH+⋅⋅⋅SeH2] complexes obtained using QTAIM calculations. Wavefunction are obtained at the B3LYP, B3PW91, and B97xD level of calculations with 6-311++G(3df,3pd) basis set.

Q

Method

ρH···Se (Ionic)

‫ߩ ׏‬ (Ionic)

ρH···Se ‫ߩ ׏‬ (Neutral) (Neutral)

ρO−H (Ionic)

‫ ߩ ׏‬ (Ionic)

ρO−H ‫ ߩ ׏‬ (Neutral) (Neutral)

−NH2

B3LYP 0.0255 B3PW91 0.0278 B97XD 0.0261

0.0090 0.0091 0.0095

0.0123 0.0130 0.0138

0.0070 0.0074 0.0080

0.3396 -0.6727 0.3367 -0.6646 0.3450 -0.6982

0.3717 0.3710 0.3751

-0.7327 -0.7298 -0.7511

−OH

B3LYP 0.0283 B3PW91 0.0308 B97XD 0.0287

0.0087 0.0086 0.0092

0.0127 0.0134 0.0144

0.0072 0.0075 0.0082

0.3321 -0.6536 0.3288 -0.6445 0.3378 -0.6810

0.3711 0.3704 0.3743

-0.7322 -0.7292 -0.7502

−CH3

B3LYP 0.0293 0.0086 B3PW91 0.0320 0.0084 B97XD 0.0299 0.0091

0.0127 0.0134 0.0142

0.0072 0.0075 0.0081

0.3288 -0.6451 0.3252 -0.6353 0.3344 -0.6726

0.3708 0.3700 0.3740

-0.7320 -0.7289 -0.7503

−H

B3LYP 0.0316 B3PW91 0.0345 B97XD 0.0322

0.0082 0.0078 0.0086

0.0129 0.0137 0.0145

0.0072 0.0076 0.0082

0.3224 -0.6277 0.3184 -0.6165 0.3285 -0.6570

0.3703 0.3694 0.3736

-0.7316 -0.7283 -0.7497

−F

B3LYP 0.0312 B3PW91 0.0340 B97XD 0.0317

0.0082 0.0079 0.0087

0.0133 0.0141 0.0148

0.0074 0.0077 0.0082

0.3237 -0.6312 0.3199 -0.6208 0.3299 -0.6607

0.3699 0.3691 0.3733

-0.7312 -0.7279 -0.7496

−Cl

B3LYP 0.030 B3PW91 0.0328 B97XD 0.0306

0.0085 0.0082 0.0089

0.0136 0.0144 0.0149

0.0075 0.0078 0.0083

0.3272 -0.6407 0.3234 -0.6303 0.3327 -0.6683

0.3692 0.3683 0.3726

-0.7305 -0.7270 -0.7491

−CN

B3LYP 0.0326 B3PW91 0.0357 B97XD 0.0334

0.0079 0.0074 0.0083

0.0146 0.0156 0.0158

0.0078 0.0082 0.0085

0.3197 -0.6201 0.3154 -0.6081 0.3250 -0.6478

0.3669 0.3657 0.3705

-0.7278 -0.7236 -0.7467

B3LYP 0.0337 −NO2 B3PW91 0.0367 b97xd 0.0351

0.0077 0.0071 0.0079

0.0150 0.0161 0.0161

0.0079 0.0083 0.0086

0.3168 -0.6117 0.3127 -0.6003 0.3208 -0.6362

0.3661 0.3649 0.3699

-0.7267 -0.7225 -0.7461

a

Values of and   are defined as e a0-3 and e a0-5, respectively.

Page 24 of 29

Table 5 Bond length (RO−H, Å), stretching frequency (ΔνO−H, cm-1), and hydrogen bond stabilization energy (ΔE, kJ mol-1) of O−H⋅⋅⋅Y hydrogen bond in [4-H−C6H4−OH+⋅⋅⋅YH2] complexes (Y = O, S, and Se) obtained at the B3LYP, B3PW91, and B97xD levels using 6311++G(3df,3pd) basis set. Δ(H-Y)a (Pauling Scale)

RO−H (complex)

ΔRO−H

νO−H (complex)

O

B3LYP B3PW91 B97xD

1.4

1.0138 1.0177 1.0064

0.0430 0.0483 0.0371

2883 2816 3004

-836 -74.07 -72.19 -928 -73.40 -71.21 -791 -77.86 -75.73

S

B3LYP B3PW91 B97xD

0.4

1.0077 1.0111 1.0010

0.0369 0.0416 0.0347

2977 2919 3090

-742 -45.80 -44.51 -825 -46.61 -45.15 -705 -49.85 -48.13

1.0075 1.0105 1.0003

0.0367 0.0410 0.0339

2979 2927 3095

-740 -44.57 -43.54 -817 -45.38 -44.19 -700 -48.47 -47.21

Y

Method

B3LYP B3PW91 Se B97xD a

0.3

Difference in electronegativity between H and Y atoms.

Page 25 of 29

ΔνO−H

ΔEHB

ΔEHB,CP

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Figure(s)

Figure 1

Figure 2

Figure 3

Figure 4

a)

b)

c)

Figure 5

Figure 6

a)

b)

c)

Figure 7

Figure 8

a)

b)

c)

Figure 9

a)

b)

c)

Figure 10

a)

b)

c)

*Highlights (for review)

► para-substituted cationic phenol−SeH2 complexes have been studied using DFT ► Electronic substituent effect on intermolecular investigated

+

O−H⋅⋅⋅Se hydrogen bond is

► Electron withdrawing substituent enhances +O−H⋅⋅⋅Se interaction energy, change in O−H bond length and stretching frequency. ► NBO, NRT, QTAIM analysis have been performed to explain effect of positive charge on O−H⋅⋅⋅Se hydrogen bond. ► Stability of +O−H⋅⋅⋅Y (Y=O, S, Se) hydrogen bonds have been investigated

Gra G aph hica al absttrac ct