Exploring the rare S—H...S hydrogen bond using charge density analysis in isomers of mercaptobenzoic acid

Abstract

Experimental and theoretical charge density analyses on isomers of mercaptobenzoic acid have been carried out to quantify the hydrogen bonding of the hitherto less explored thiols, to assess the strength of the interactions using the topological features of the electron density. The electron density study offers interesting insights into the nature of the S-H...S interaction. The interaction energy is comparable with that of a weak hydrogen bond. The strength and directionality of the S-H...S hydrogen bond is demonstrated to be mainly due to the conformation locking potential of the intramolecular S...O chalcogen bond in 2-mercaptobenzoic acid and is stronger than in 3-mercaptobenzoic acid, which lacks the intramolecular S...O bond. The para-substituted mercaptobenzoic acid depicts a type I S...S interaction.

Mapping of N−C Bond Formation from a Series of Crystalline Peri‐Substituted Naphthalenes by Charge Density and Solid‐State NMR Methodologies

“…Given that the typical charge density and Laplacian values for asingle CÀCbond are ca. 1.6 e À3 and À10 to À12 e À5 ,this 15 Nand 13 Cisotropic chemical shifts, and the GIPAW-DFT calculated 1 J NC couplings and isotropic shifts. The DFT isotropic shifts were determined using d iso = À[s cal Às ref ], where s ref = 170 ppm for 13 Cand À153 ppm for 15 N. [19] Charge Density Experimental MAS NMR [20] 2.292(4) 2.245 0.33 2.26 --2.292 0.61 À314 183 S2 [21] 1.993( 3 ------2.167 1.94 --100 K [8] ------1.750 7.04 --…”

Mapping of N−C Bond Formation from a Series of Crystalline Peri‐Substituted Naphthalenes by Charge Density and Solid‐State NMR Methodologies

“…1.6 e À3 and À10 to À12 e À5 ,this 15 Nand 13 Cisotropic chemical shifts, and the GIPAW-DFT calculated 1 J NC couplings and isotropic shifts. The DFT isotropic shifts were determined using d iso = À[s cal Às ref ], where s ref = 170 ppm for 13 Cand À153 ppm for 15 N. [19] Charge Density Experimental MAS NMR [20] 2.292(4) 2.245 0.33 2.26 --2.292 0.61 À314 183 S2 [21] 1.993( 3 ------2.167 1.94 --100 K [8] ------1.750 7.04 -suggests that the double bond in 6 has not been fully transformed into as ingle (s)b ond. [22] There is also ac hange in the ellipticity of this bond, derived from the CD determination (Section 2, ESI), from 0.29-0.32 for 1 and 3,t o0 .13-0.17 for 4-6 which could be interpreted as areduction in the p component of the bonding,t hough this approach has been questioned since there is no direct connection between the topological analysis and an orbital based description of bonding.…”

“…Thec harge density,d etermined from X-ray diffraction measurements,m aps the valence electron distributions between the interacting groups.T he corresponding crystal structures for S1-S2 and 1-6 have been used to calculate, via DFT,t he 1 J NC couplings between 15 Na nd 13 Ca toms located at either end of the peri interaction/partial bond to characterise the interaction across the Me 2 N•••C bridge.T his use of DFT to predict the NMR parameters is compared to experimental measurements made on naphthalenes 2, 3, 4 and 6 in which the two interacting/bonding atoms are both isotopically labelled (Scheme S1, ESI).…”

“…[28] Amechanism for measuring the 1 J coupling in the solid-state is to utilise the spin-echo sequence which refocuses the evolution of all the terms that appear as offsets,inparticular those that are caused by adistribution of chemical shifts.T he use of spin echoes in an NMR experiment allows the detection of chemical shift separated J couplings even when inhomogeneous broadening means it is not directly visible in the observed spectrum. [29] Thep resence of other highly coupled nuclei with large quadrupolar couplings (such as 14 No r 17 O) can also cause dephasing of the signal and prevent accurate measurements of the J-coupling,t herefore the measurement was observed from the perspective of the 15 Nnuclei which is solely coupled to 99 % 13 C(I = 1 = 2 ), whereas the more sensitive 13 Ciscoupled to 60 % 15 N( I = 1 = 2 )a nd 40 %q uadrupolar 14 N( I = 1), which would cause greater dephasing and overestimation of the 1 J coupling.W eh ave previously discussed am ethodology of utilizing periodic DFT calculations on molecular structures determined by single-crystal X-ray crystallography to determine 1 J couplings which are validated with spin-echo solidstate NMR experiments.H ere,w ee xpand this methodology to compare the charge density maps with NMR observations. [29] Theh eteronuclear 15 N- 13 Cs pin-echo decay for the aldehyde 2 and dinitrile 3 (Figure 2a and b) both show very shallow exponential decays which are indicative of narrow resonances that are not broadened by 1 J NC coupling contributions.Agreen simulated fit is given for the T 2 decays,with the blue SIMPSON [30] simulation of the periodic DFT determined 1 J NC coupling,a nd their product is given in red.…”

“…Them ost closely-related charge density studies on peri-systems relate to proton sponges and al ong (2.700 )M e 2 N•••CONMe 2 interaction, whilst investigations into Cl•••Cl interactions and extended hypervalent bonding in S•••S À S•••S and S•••Se À Se•••S situations have also been completed. [14,15] Solid-state 1 J coupling studies between peri chalcogenide elements have also been made. [16] However, no combined charge density/ 1 Jcoupling studies on crystalline peri-naphthalenes have been reported.…”