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In the current research, the performance of molecular tailoring approach (MTA) for estimation of the intramolecular hydrogen bond (IMHB) energies of the simple resonance-assisted hydrogen bond (RAHB) systems was theoretically investigated. First, a wide range of malonaldehyde derivatives (36 members) including the F, Cl, Br, CN, NO2, ethen (-CH=CH2), ethin (-C ≡ CH), CF3, OCH3, C2H5, CH3, and Ph substitutions at R1, R2, and R3 positions were considered. Then, all of these molecules at MP2/6-311++G(d, p) level of theory have been optimized and their MTA energies were calculated. Furthermore, various qualitative descriptors of IMHB such as structural, spectroscopic, topological, and molecular orbital parameters were considered, and all of correlations between these factors and MTA energies were explored. According to their regression coefficients (R2), the linear characteristic of correlations obeys the following order: H H ... O > d O − H > E CT > V H • • • O > ρ H • • • O > ν O − H > d H • • • O > d O • • • O 0.952 0.940 0.936 0.914 0.901 0.834 0.789 0.755 These correlation coefficients have compared with the corresponding R2 values of other models such as RRM, RBM, GCM, IRM, and OCM, which leads to the following order of linearity: MTA ≥ RRM > RBM > GCM > IRM > OCM. Finally, the significance of π-electron delocalization (π-ED) of RAHB rings is also evaluated by the geometrical factor of Gilli (λ) and the harmonic oscillator model of aromaticity (HOMA) that presents the excellent linear correlations with MTA energies, which may be implied on the validity of RAHB theory. [ABSTRACT FROM AUTHOR]

O, S, Se, and P atoms are considered as different proton donor/acceptor moieties in the resonance-inhibited hydrogen bonded (RIHB) interactions to conduct a comparative investigation. In order to study the effects of resonance and σ-skeleton planarity, the corresponding saturated and planar molecules are also computed. Moreover, to provide a broader perspective, all the given systems are compared to their resonance-assisted hydrogen bonded (RAHB) analogues. The natural bond orbital (NBO), the quantum theory of atoms in molecule (QTAIM), and the gauge-independent atomic orbital (GIAO) analyses, are performed to estimate the strength and nature of the intramolecular hydrogen bonds (IMHBs). The strength of IMHBs is also estimated using energy differences between closed and open conformers. According to the results, all the given RIHB systems suggest that resonance can obviously decrease the strength of the IMHBs. However, in the RIHB systems and also in some RAHB ones, the role of π-conjugation in the IMHB formation is less important than that of σ-planarity. In other words, the constraints imposed by σ-skeleton cause both the H-bond acceptor and donor groups to be situated close to each other; these effects outweigh the undesirable results of resonance, which totally lead to the formation of the RIHB. [ABSTRACT FROM AUTHOR]

In the present study, the conformational analysis, intramolecular hydrogen bond, π-electron delocalization, and tautomeric equilibriums in 2-selenoformyl-3-thioxo-propionaldehyde (STP) were investigated. First, conformational analysis of STP was performed, and our results have shown that the chelated thiol structures are more stable than the chelated enol and selenol ones; HB Thiol < HB Enol < HB Selenol. To justify this order, the affective factors on this stability such as tautomerism, hydrogen bonding, and π-electron delocalization were comprehensively investigated. Survey of tautomeric equilibriums in STP indicates that destabilization energy of keto⇋enol equilibrium (about 50 kJ/mol) is more than the seleno⇋selenol (about 22 kJ/mol) and then thio⇋thiol (about 20 kJ/mol) cases. Description of different hydrogen bonds by energetic, geometrical, topological, and molecular orbitals emphasizes on the presence of a stronger intramolecular hydrogen bond (IMHB) in the enol forms. Furthermore, evaluation of π-electron delocalization (π-ED) represents approximate superiority of enol conformers. The achieved results by HB and π-ED analyzes are in contrast with the mentioned stability order. Ultimately, investigation of above factors indicates that the tautomerism phenomenon plays a dominant role in determining of the conformational preferences. [ABSTRACT FROM AUTHOR]

A comprehensive theoretical investigation of the mutual effects of the cation-π, anion-π, and intramolecular hydrogen bond (IMHB) interactions in the various ternary complexes of 1,3,5-triamino-2,4,6-trinitrobenzene (ANB) with anions (A: F, Cl, Br) and cations (M: Li, Na, K, Mg, Ca) were carried out. The energetic, geometrical, topological, and molecular orbital descriptors are employed to estimate the strength of the mentioned non-covalent interactions and their results were compared with the corresponding data of the ANB⋯M, ANB⋯A and A⋯Bz⋯M complexes, as a set of reference points. The results indicate that the presence of resonance-assisted hydrogen bond (RAHB) units decreases the strength of cation-π interaction while for anion-π interactions, the reverse process is observed. On the other hand, the RAHB units reduce the binding energies of the ternary complexes and cooperative effects between the cation-π and anion-π interactions, with respect to the A⋯Bz⋯M complexes. Furthermore, the coexistence of cation-π and anion-π interactions modulates their effects on the strength of IMHB. Finally, the harmonic oscillator model of aromaticity (HOMA) indicator was applied to analyze the influences of the mentioned interactions on the significance of the π-electron delocalization (π-ED) of the RAHB units and aromaticity of the benzene ring. It was found that the coexistence of cation-π and anion-π interactions effectively change the π˗ED and aromaticity. [ABSTRACT FROM AUTHOR]

A comprehensive theoretical study of mutual interactions between the resonance-assisted hydrogen bond (RAHB) units and benzene rings in salicylaldehyde and ortho-aminobenzaldehyde with their thio and seleno analogues, at the M062X/6-11++G(d,p) level of theory, was carried out. First, we evaluated the intramolecular hydrogen bond (IMHB) strength of the mentioned compounds by various descriptors, such as geometrical, spectroscopic, topological, molecular orbital, and energetic parameters, and then compared with the corresponding results of malonaldehyde and β-aminoacrolein with their thio and seleno analogues. According to the theoretical results, it was found that the merging of benzene rings and RAHB units reduced the strength of the IMHB in all of the benchmark systems. The magnitude of these reductions in the O-H⋯Y systems is greater than that of the N-H⋯Y ones. Also, the substitution of S and Se atoms instead of O increased the IMHB strength and obeyed the following order; X-H⋯Se > X-H⋯S > X-H⋯O. Second, a set of aromaticity indices such as the harmonic oscillator model of aromaticity (HOMA), the nucleus-independent chemical shift (NICS), the para delocalization index (PDI), and the average two-center index (ATI) were applied to evaluate the significance of the aromaticity of benzene rings and π-ED of RAHB units in the studied molecules. The comparison between these results and the respective amounts of the benzene ring, malonaldehyde, and β-aminoacrolein analogues reveals that the coupling of benzene and RAHB rings decreased the π-ED and aromaticity in both of them. These reductions approximately increase with the replacement of O by N in the proton donor group. Finally, the results show that the π-ED of RAHB units increases with substitution of O by S and Se, while for the aromaticity of the benzene rings the reverse process is observed. The majority increment in the π-ED of RAHB units and also a reduction in the aromaticity of the benzene rings are related to an X-H⋯Se system that is followed by X-H⋯S and X-H⋯O systems. [ABSTRACT FROM AUTHOR]

We first use the second-order Møller-Plesset theory to optimize β-aminoacrolein, β-thioaminoacrolein and a set of their halogenated derivatives, which involve N-H···O and N-H···S hydrogen bonds. Then, four different methods, namely rotation barrier method (RBM), isodesmic reaction method (IRM), geometry corrected method (GCM) and related rotamers method, (RRM) are utilized to estimate the intramolecular hydrogen bond (IMHB) energies. The linear correlations between the estimated IMHB energies and some hydrogen bond strength descriptors such as geometrical, electron density topological and spectroscopic parameters are obtained. According to our data, RRM and RBM have the best linear correlations with all of the hydrogen bond descriptors, while GCM and IRM do not reveal suitable results. It is assumed that the performance or reliability of the estimating methods is determined based on the value of squared regression coefficients ( R ). By adding the results of the previous studies including OH···O, OH···S and NH···O hydrogen bond units to the present obtained data, the methods can be sorted according to their relative reliability as follows: RRM > RBM ≫ GCM > IRM. [ABSTRACT FROM AUTHOR]

In the present study, for the first time, the first singlet excited state (S1) intramolecular hydrogen bond (IMHB) of malonaldehyde derivatives by time-dependent density functional theory (TD-DFT) and configuration interaction singles (CIS) with 6-311++G( d, p) basis set is investigated. In this regard, we initially characterized the equilibrium structures of the first singlet excited states of the benchmark systems and some of their properties such as excitation energies, oscillator strengths and dipole moments were analyzed. Then, the IMHB energies have been calculated by several adopted methods, such as potential energy density (PED) of Espinosa, related rotamers method (RRM) and rotational barrier method (RBM). The RBM and RRM energies appear grossly overestimate the intramolecular hydrogen bond energies as compared to the PED method and are not convenient for estimation the IMHB energy of excited states. Then, the more reliable PED results were used to systematically investigate the halogen substitution effects. It was found that the halogen replacement at R3 position decreases the IMHB energies, both in the ground and the first singlet excited states, while the halogen substitution at R1 and R2 positions of ground and excited states have paradoxical effects on the IMHB energies. Subsequently, the geometrical, topological, molecular orbital and spectroscopic descriptors of IMHB are comprehensively examined and various correlations between the PED energies and different HB descriptors are explored. Our results show that the excellent linear correlations between the PED energies and the majority of HB descriptors exist. Interestingly, the linear character of correlations between the HB descriptors with CIS energies is more than the TD-DFT ones. Consequently, on the basis of these correlations, one can claim that the PED method, E ≈ V( r )/2, is a trusty equation for estimation of IMHB energy of RAHB systems, both in the ground and first singlet excited states (S1). [ABSTRACT FROM AUTHOR]