Structural and vibrational spectroscopic elucidation of sulpiride in solid state

The study on the conformational and vibrational behaviors of sulpiride molecule which is known as a neuroleptic or antipsychotic drug that is widely used clinically in the treatment of schizophrenic or depressive disorders is an important scientific and practical task. In here, a careful enough study of monomer and dimeric forms of sulpiridine {5-(aminosulfonyl)-N-[(1-ethyl-2-pyrrolidinyl) ethyl]-2-methoxy-benzamide (C₁₅H₂₃N₃O₄S)} is undertaken by density functional theory (DFTB3LYP) method with the B3LYP/6-31G(d,p) basis set. The conformations of free molecule were searched by means of torsion potential energy surfaces scan studies through dihedral angles D₁ (8?N, 18C, 20C, 23?N), D₂ (18C, 20C, 23?N, 25C) and D₃ (28C, 30C, 41S, 44?N) in electronically ground state, employing 6-31G basic set. The final geometrical parameters for the obtained stable conformers were determined by means of geometry optimization, carried out at DFT/B3LYP/6-31G(d,p) theory level. Afterwards, the possible dimer forms of the molecule were formed and their energetically preferred conformations were investigated. Moreover, the effect of basis set superposition error on the structure and energy of the three energetically favourable sulpiride dimers has been determined. The optimized structural parameters of the most stable monomer and three low energy dimer forms were used in the vibrational wavenumber calculations. Raman and IR (4000–400?cm^?1) spectra of sulpiride have been recorded in the solid state. The assignment of the bands was performed based on the potential energy distribution data. The natural bond orbital analysis has been performed on both monomer and dimer geometries in order to elucidate delocalization of electron density within the molecule. The predicted frontier molecular orbital energies at DFT/B3LYP/6-31G(d,p) theory level show that charge transfer occurs within the molecule. The first-order hyperpolarizability (β₀) and related properties (μ and α) of the title molecule were also calculated.     

An Investigation of the Structural Diversities of Lithiated HMPA Complexes of o-Mercaptopyridine and Trithiocyanuric Acid: Syntheses, Crystal structures and Model Molecular Orbital Calculations

Reaction of o-mercaptopyridine (o-MPH) and trithiocyanuric acid (TTCyH₃) with one equivalent of BuⁿLi in the presence of HMPA yields the mono-lithiated salts MPLi.HMPA (1) and TTCyH₂Li.2HMPA (2) respectively, which have been characterised by NMR spectroscopy and X-ray crystallography. Reaction of three equivalents of BuⁿLi with anhydrous TTCyH₃ in THF yields the tri-lithiated species TTCyLi₃.4THF (3). In all three compounds the lithium centres have N,S-bridged coordination modes. Whereas 1 is dimeric in the solid state, 2 has an unusual monomeric structure and 3, which is a very rare example of a structurally characterised tri-lithiated compound, has an unprecedented polymeric structure incorporating (NCSLi) _n (n = 1, 2) rings. The structural diversities displayed by 1 and 2 have been probed, and thereby in part rationalised, by ab initio (6-31G*/RHF, 6-31G**/RHF and 6-31G*/MP2 levels) MO calculations on both their thio-keto and thiol isomers and on their uncomplexed and complexed lithiated derivatives. In particular, the optimised structures predict and reproduce the N,S-bridging coordination modes found for lithium and explain why structure 1 is dimeric whereas 2 is monomeric.Electronic Supplementary Material available.     

Fast kinetics of nucleotide binding to Clostridium perfringens family II pyrophosphatase containing CBS and DRTGG domains

We earlier described CBS-pyrophosphatase of Moorella thermoacetica (mtCBS-PPase) as a novel phosphohydrolase that acquired a pair of nucleotide-binding CBS domains during evolution, thus endowing the protein with the capacity to be allosterically regulated by adenine nucleotides (J?msen, J., Tuominen, H., Salminen, A., Belogurov, G. A., Magretova, N. N., Baykov, A. A., and Lahti, R. (2007) Biochem. J., 408, 327–333). We herein describe a more evolved type of CBS-pyrophosphatase from Clostridium perfringens (cpCBS-PPase) that additionally contains a DRTGG domain between the two CBS domains in the regulatory part. cpCBS-PPase retained the ability of mtCBS-PPase to be inhibited by micromolar concentrations of AMP and ADP and activated by ATP and was additionally activated by diadenosine polyphosphates (AP _n A) with n > 2. Stopped-flow measurements using a fluorescent nucleotide analog, 2′(3′)-O-(N-methylanthranoyl)-AMP, revealed that cpCBS-PPase interconverts through two different conformations with transit times on the millisecond scale upon nucleotide binding. The results suggest that the presence of the DRTGG domain affords greater flexibility to the regulatory part, allowing it to more rapidly undergo conformational changes in response to binding.     

A MP2 and DFT study of the aromatic character of polyphosphaphospholes. Is the pyramidality the only factor to take into consideration?

A comprehensive MP2/6-311 + G(d,p) and B3LYP/6-311 + G(d,p) study of the aromatic character of phospholes, P _n (CH)_4-n PH with n = 0-4 was conducted. For this purpose, the structures for these compounds were optimized at both theoretical levels and different magnetic properties (magnetic susceptibility anisotropy, χ_anis, and the nucleus-independent chemical shifts, NICS) were evaluated. For comparison, these magnetic properties were also calculated in the optimized structures with planarity constraints. We have also applied the ACID (anisotropy of the current-induced density) method in this analysis. The main conclusions are the aromatic character of these compounds, the relationship between aromaticity and planarity and the importance of other factors in this aromaticity.     

Complete basis set,hybrid-DFT study and NBO interpretation of conformational analysis of 2-methoxytetrahydropyran and its thiopyran and selenopyran analogues in relation to the anomeric effect

2-Methoxytetrahydropyran (1), -thiopyran (2) and -selenopyran (3) have been chosen as model compounds to investigate the origin of the anomeric effect (AE). The impacts of the hyperconjugation, electrostatic and steric interactions on the conformational preferences of compounds 1–3 have been analysed by means of complete basis set-4, hybrid-density functional theory (B3LYP/6-311+G**) based methods and natural bond orbital (NBO) interpretation. Both levels of theory showed that the axial conformations of compounds 1–3 are more stable than their equatorial conformations. The Gibbs free energy difference (G _eq–G _ax) values (i.e. ΔG _eq–ax) between the axial and equatorial conformations increase from compound 1 to compound 2 but decrease from compound 2 to compound 3. Based on the NBO results obtained, the AE associated with the electron delocalisation [i.e. Σ(endo-AE_eq + exo-AE_eq) ? Σ(endo-AE_ax + exo-AE_ax)] increase slightly from compound 1 to compound 2 but decrease from compound 2 to compound 3. Similar trend is also observed for the differences between the calculated total steric exchange energy values [i.e. Δ(TSEE)_eq–ax]. On the other hand, the calculated differences between the dipole moment values of the axial and equatorial conformations [i.e. Δ(μ_eq–μ_ax)] decrease from compound 1 to compound 3. These findings led to the proposal that the AE associated with the electron delocalisation (the hyperconjugation effect) is more significant for the explanation of the conformational preferences of compounds 1–3 than the electrostatic model. The correlations between the AE associated with the electron delocalisation, bond orders, TSEE, ΔG _eq–ax, dipole–dipole interactions, structural parameters and conformational behaviours of compounds 1–3 have been investigated.     

New dinuclear copper(II) and zinc(II) complexes for the investigation of sugar–metal ion interactions

We have studied the binding interactions of biologically important carbohydrates (d-glucose, d-xylose and d-mannose) with the newly synthesized five-coordinate dinuclear copper(II) complex, [Cu₂(hpnbpda)(μ-OAc)] (1) and zinc(II) complex, [Zn₂(hpnbpda)(μ-OAc)] (2) [H₃hpnbpda = N,N′-bis(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine-N,N′-diacetic acid] in aqueous alkaline solution. The complexes 1 and 2 are fully characterized both in solid and solution using different analytical techniques. A geometrical optimization was made of the ligand H₃hpnbpda and the complexes 1 and 2 by molecular mechanics (MM+) method in order to establish the stable conformations. All carbohydrates bind to the metal complexes in a 1:1 molar ratio. The binding events have been investigated by a combined approach of FTIR, UV–vis and ¹³C NMR spectroscopic techniques. UV–vis spectra indicate a significant blue shift of the absorption maximum of complex 1 during carbohydrate coordination highlighting the sugar binding ability of complex 1. The apparent binding constants of the substrate-bound copper(II) complexes have been determined from the UV–vis titration experiments. The binding ability and mode of binding of these sugar substrates with complex 2 are indicated by their characteristic coordination induced shift (CIS) values in ¹³C NMR spectra for carbon atoms C1, C2, and C3 of sugar substrates.     

Density functional theory molecular modelling,DNA interactions,antioxidant, antimicrobial,anticancer and biothermodynamic studies of bioactive water soluble mixed ligand complexes

A novel series of bioactive water soluble mixed ligand complexes (1–5) [M^II(L)(phen)AcO]. nH₂O {where M?=?Cu (1) n?=?2; Co (2), Mn (3), Ni (4), n?=?4 and Zn (5) n?=?2} were synthesized from 2-(2-Morpholinoethylimino) methyl)phenol Schiff base ligand (LH), 1, 10-phenanthroline and metal(II) acetate salt in a 1:1:1 stoichiometric ratio and characterized by several spectral techniques. The obtained analytical and spectral data suggest the octahedral geometry around the central metal ion. Density functional theory calculations have been further supportive to explore the optimized structure and chemical reactivity of these complexes from their frontier molecular orbitals. Gel electrophoresis result indicates that complex (1) manifested an excellent DNA cleavage property than others. The observed binding constants with free energy changes by electronic absorption technique and DNA binding affinity values by viscosity measurements for all compounds were found in the following order (1)?>?(2)?>?(4)?>?(5)?>?(3) > (LH). The binding results and thermodynamic parameters are described the intercalation mode. In vitro antioxidant properties disclose that complex (1) divulges high scavenging activity against DPPH^?, ^?OH, O₂^?? NO^?, and Fe³⁺. The antimicrobial reports illustrate that the complexes (1–5) were exhibited well defined inhibitory effect than ligand (LH) against the selected different pathogenic species. The observed percentage growth inhibition against A549, HepG2, MCF-7, and NHDF cell lines suggest that complex (1) has exhibited superior anticancer potency than others. Thus, the complex (1) may contribute as potential anticancer agent due to its unique interaction mode with DNA.GRAPHICAL ABSTRACT

Communicated by Ramaswamy H. Sarma     

UV Photoelectron Spectroscopy and Ab Initio Characterization of Valence Orbital Structures and Conformations of Neutral Phosphate Esters

Abstract

The Hel UV photoelectron spectrum of trimethyl phosphate (TMP) has been measured and interpreted with the aid of SCF molecular orbital calculations carried out with STO-3G, STO-3G* and 4–31G basis functions. The photoelectron spectrum of TMP is more accurately reproduced by results from 4–31G calculations than by results from STO-3G or STO-3G* calculations. However, all three basis sets yield results which predict the same assignment of the photoelectron spectrum. Results at the 4–31G level indicate that whether calculations are based on crystallographic bond angles and bond lengths or on STO-3G optimized geometries has little effect on the energetic ordering of the upper occupied orbitals. The energetic ordering of orbitals is also found to be only weakly dependent upon the torsional angle φ, describing rotation of ester groups about P-O bonds and upon the torsional angle ψ, describing rotation of methyl groups about C-O bonds. For trimethyl phosphate, with C₃ symmetry, the vertical ionization potentials of the upper occupied orbitals are 10.81 eV (8e), 11.4 eV (9a), 11.93 eV (7e), 12.6–12.9 eV (8a and 6e), 14.4 eV (7a) and 15.0–16.0 eV(5e and 6a). Calculations at the 4–31G level indicate that many of the highest occupied orbitals in neutral dimethyl phosphate and methyl phosphate have energies and electron distributions similar to orbitals in TMP.

For TMP, a search for optimized values of φ and ψ has been carried out at the STO-3G* level. In agreement with previous NMR studies and with classical potential calculations, the STO- 3G* results indicate that both the gauche φ= 53.1 °) and anticlinal (φ = 141.9°) conformations are thermally accessible. Also in agreement with the classical potential calculations, the STO-3G* results predict that in the all gauche conformation energy is minimized when the methyl groups assume a staggered geometry (ψ= 60° to 80°) and that an energy maximum occurs for an eclipsed geometry (ψ = 0° to 20°). A study of the dependence of optimized values of O-P-O ester bond angles on the torsional angles, φ, was carried out at the STO-3G, STO-3G* and 4–31G levels. The results demonstrate that for C₃ symmetry, the coupling of O-P-O angles to φ is influenced by repulsive steric interactions.     

3D-QSAR studies of triazolopyrimidine derivatives of <Emphasis Type="Italic">Plasmodium falciparum</Emphasis> dihydroorotate dehydrogenase inhibitors using a combination of molecular dynamics,docking, and genetic algorithm-based methods

A series of 35 triazolopyrimidine analogues reported as Plasmodium falciparum dihydroorotate dehydrogenase (PfDHODH) inhibitors were optimized using quantum mechanics methods, and their binding conformations were studied by docking and 3D quantitative structure–activity relationship studies. Genetic algorithm-based criteria was adopted for selection of training and test sets while maintaining structural diversity of training and test sets, which is also very crucial for model development and validation. Both the comparative molecular field analyses ( q_\textLOO² = 0.841 q_{\text{LOO}}^2 = 0.{841} , r_\textncv² = 0.99 r_{\text{ncv}}^2 = 0.{99} ) and comparative molecular similarity indices analyses ( q_\textLOO² = 0.757 q_{\text{LOO}}^2 = 0.{757} , r_\textncv² = 0.943 r_{\text{ncv}}^2 = 0.{943} ) show excellent correlation and high predictive power. Furthermore, molecular dynamics simulations were performed to explore the binding mode of the two of the most active compounds of the series, 10 and 14. Harmonization in the two simulation results validate the analysis and therefore applicability of docking parameters based on crystallographic conformation of compound 14 bound to receptor molecule. This work provides useful information about the inhibition mechanism of this class of molecules and will assist in the design of more potent inhibitors of PfDHODH.     

Complexes of DNA bases and Watson–Crick base pairs interaction with neutral silver Agn (n = 8, 10, 12) clusters: a DFT and TDDFT study

We study the binding of the neutral Ag_n (n = 8, 10, 12) to the DNA base-adenine (A), guanine (G) and Watson–Crick –adenine-thymine, guanine-cytosine pairs. Geometries of complexes were optimized at the DFT level using the hybrid B3LYP functional. LANL2DZ effective core potential was used for silver and 6–31 + G^** was used for all other atoms. NBO charges were analyzed using the Natural population analysis. The absorption properties of Ag_n–A,G/WC complexes were also studied using time-dependent density functional theory. The absorption spectra for these complexes show wavelength in the visible region. It was revealed that silver clusters interact more strongly with WC pairs than with isolated DNA complexes. Furthermore, it was found that the electronic charge transferred from silver to isolated DNA clusters are less than the electronic charge transferred from silver to the Ag_n–WC complexes. The vertical ionization potential, vertical electron affinity, hardness, and electrophilicity index of Ag_n–DNA/WC complexes have also been discussed.     

研究报告: 基于遗传算法的计算机辅助策略优化孕酮适配体

目的 本研究致力于优化孕酮（progesterone，P4）适配体的亲和力和选择性。方法 基于遗传算法（genetic algorithm，GA）的计算机辅助优化策略（in silico maturation，ISM），进行了4轮GA操作（含交叉变异、单点突变和双点突变操作），构建了初始文库和G1、G2、G3代ssDNA作为新的候选适配体库，采用分子对接对候选适配体进行筛选和分析，并使用迭代策略不断优化适配体。此外，还提出了一种较为准确预测ssDNA三级结构的方法，首先使用Mfold预测二级结构，继而使用RNAComposer建立与ssDNA相对应的RNA三级结构，输出的PDB文件使用Discovery Studio将RNA修改为DNA，最后使用Molecular Operating Environment对结构进行能量最小化处理。结果 到G2代，在局部搜索空间对P4S-0进行优化，筛选出P4G1-14、P4G2-20、P4G1-6、P4G1-7和P4G2-14这5条适配体作为P4的最佳候选适配体。采用AuNPs比色法初步验证优化后适配体的亲和力，继而构建了基于适配体结构开关的荧光法测定适配体的解离常数（equilibrium dissociation constant，K_D），并以此方法对适配体的选择性（对双酚A、雌二醇、睾酮和皮质醇）进行了评估。结论 通过ISM优化后的适配体，对P4的亲和力较原适配体有了较大提升，仍保留着识别结构类似分子的选择性。     

VCD Robustness of the Amide‐I and Amide‐II Vibrational Modes of Small Peptide Models

The rotational strengths and the robustness values of amide‐I and amide‐II vibrational modes of For(AA)_nNHMe (where AA is Val, Asn, Asp, or Cys, n = 1–5 for Val and Asn; n = 1 for Asp and Cys) model peptides with α‐helix and β‐sheet backbone conformations were computed by density functional methods. The robustness results verify empirical rules drawn from experiments and from computed rotational strengths linking amide‐I and amide‐II patterns in the vibrational circular dichroism (VCD) spectra of peptides with their backbone structures. For peptides with at least three residues (n ≥ 3) these characteristic patterns from coupled amide vibrational modes have robust signatures. For shorter peptide models many vibrational modes are nonrobust, and the robust modes can be dependent on the residues or on their side chain conformations in addition to backbone conformations. These robust VCD bands, however, provide information for the detailed structural analysis of these smaller systems. Chirality 27:625–634, 2015 © 2015 Wiley Periodicals, Inc.     

Effect of Flanking Sequences on the Right- to Left- Handed Transition of a (dA-dT)n Tract in Supercoiled DNA

Abstract

Alternating (dA-dT)_n sequences in supercoiled DNA may undergo a transition to a left-handed conformation in the presence of Ni²⁺ ions and high NaCl concentration (Nejedlý, K., Klysik, J. and Pale?ek, E, FEBS Lett. 243, 313–317 (1989)). In this work we have found that ionic conditions necessary for the B-to-Z transition are strongly dependent on the sequences flanking the (dA-dT)_n tract. In particular, the presence of 5′- homopyrimidine (C3) and 3′-homopurine (G4) blocks adjacent to the tract were found to facilitate the transition to the left- handed form. Within a constant sequence context it was found that the ionic strength required to promote the transition was inversely proportional to the length of the (dA-dT) _n sequence.     

Poloxamer 407-based intranasal thermoreversible gel of zolmitriptan-loaded nanoethosomes: formulation,optimization, evaluation and permeation studies

Zolmitriptan is the drug of choice for migraine, but low oral bioavailability (<50%) and recurrence of migraine lead to frequent dosing and increase in associated side effects. Increase in the residence time of drug at the site of drug absorption along with direct nose to brain targeting of zolmitriptan can be a solution to the existing problems. Hence, in the present investigation, thermoreversible intranasal gel of zolmitriptan-loaded nanoethosomes was formulated by using mucoadhesive polymers to increase the residence of the drug into the nasal cavity. The preparation of ethosomes was optimized by using 3² factorial design for percent drug entrapment efficiency, vesicle size, zeta potential, and polydispersity index. Optimized formulation E6 showed the vesicle size (171.67?nm) and entrapment efficiency (66%) when compared with the other formulations. Thermoreversible gels prepared by using poloxamer 407 showed the phase transition temperature at 32–33?°C which was in line with the nasal physiological temperature. The optimized ethosomes were loaded into the thermoreversible mucoadhesive gel optimized by varying concentrations of poloxamer 407, carbopol 934, HPMC K100, and evaluated for gel strength, gelation temperature, mucoadhesive strength, in vitro drug release, and ex vivo drug permeation, where G3 and G6 were found to be optimized formulations. In vitro drug release was studied by different kinetic models suggested that G3 (n?=?0.582) and G6 (n?=?0.648) showed Korsemeyer–Peppas (K_KP) model indicating non-Fickian release profiles. A permeation coefficient of 5.92 and 5.9?µg/cm² for G3 and G6, respectively, revealed very little difference in release rate after 24?h between both the formulations. Non-toxic nature of the gels on columnar epithelial cells was confirmed by histopathological evaluation.     

A Theoretical Model for the Binding of Cis-Pt(NH3)2 +2 to DNA

Abstract

The binding of cis-Pt(NH₃)₂B₁B2 to the bases B₁ and B₂, i.e., guanine (G), cytosine (C), adenine (A), and thymine (T), of DNA is studied theoretically. The components of the binding are analyzed and a model structure is proposed for the intrastrand binding to the dB,pdB₂ sequence of a kinked double helical DNA. Quantum mechanical calculations of the ligand binding energy indicates that cw-Pt(NH₃)₂ ⁺² (cis-PDA) binds to N7(G), N3(C), 02(C), 06(G), N3(A), N7(A), 04(T) and 02(T) in order of decreasing binding energy. Conformational analysis provides structures of kinked DNA in which adjacent bases chelate to cis-PDA. Only bending toward the major groove allows the construction of acceptable square planar complexes. Examples are presented for kinks of ?70° and ?40° at the receptor site to orient the base pairs for ligand binding to B, and B₂ to form a nearly square planar complex. The energies for complex formation of cis-PDA to the various intra-strand base sites in double stranded DNA are estimated. At least 32 kcal/mole separates the energetically favorable dGpdG·cis-PDA chelate from the dCpdG·cis-PDA chelate. All other possible chelate structures are much higher in energy which correlates with their lack of observation in competition with the preferred dGpdG chelate.

The second most favorable ligand energy occurs with N3(C). A novel binding site involving dC(N3)pdG(N7) is examined. Denaturation can result in an anti ? syn rotation of C about its glycosidic bond to place N3(C) in the major groove for intrastrand binding in duplex DNA. This novel intrastrand dCpdG complex and the most favored dGpdG structure are illustrated with stereographic projections.     

A multispecific monoclonal antibody G2 recognizes at least three completely different epitope sequences with high affinity

A monoclonal antibody (mAb) G2 possesses an unusual characteristic of reacting with at least three proteins (ATP6V1C1, SEPT3, and C6H10orf76) other than its original antigen, chicken prion protein (ChPrP). The epitopes on ChPrP and ATP6V1C1 have been identified previously. In this study, we identified the epitope in the third protein, SEPT3. Interestingly, there was no amino acid sequence similarity among the epitopes on the three proteins. These epitopes had high binding affinities to G2 (K _D = ～10^?7 M for monovalent binding and K _D = ～10^?9 M for divalent binding), as determined using a SPR biosensor. This is the first report on a three‐in‐one mAb recognizing completely different epitope sequences with high affinity. Additionally, competitive ELISA indicated that the binding sites on G2, specific for the three different epitopes, overlapped, suggesting that the antigen‐binding site may be flexible in the free form and capable of adapting to at least three different conformations to enable interactions with three different antigens.     

Theoretical study on the structural,vibrational, and thermodynamic properties of the (Br<Subscript>2</Subscript>GaN<Subscript>3</Subscript>)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 1–4) clusters

The molecular geometries, vibrational properties, and thermodynamic properties of the clusters (Br₂GaN₃) _n (n = 1–4) were studied at the B3LYP/6-311+G* level. The optimized clusters (Br₂GaN₃) _n (n = 2–4) were all found to possess a cyclic structure consisting of Ga atoms bridged by the α-nitrogen of the azide groups. A discussion of the relationships between the geometrical parameters and the degree of oligomerization n is provided. Features in the IR spectra were assigned by vibrational analysis. Trends in thermodynamic properties with temperature and degree of oligomerization n are discussed. Thermodynamic analysis of the gas-phase reaction showed that the formation of the clusters (Br₂GaN₃) _n (n = 2–4) is thermodynamically favorable considering the enthalpies at 298.2 K. The calculated results for the Gibbs free energies were negative, which indicates that the oligomerizations can occur spontaneously at 298.2 K.     

Ab initio calculation of the geometries, stabilities, and electronic properties for the bimetallic Be2Au(n) (n = 1-9) clusters: comparison with pure gold clusters

Ab initio methods based on density functional theory at BP86 level were applied to the study of the geometrical structures, relative stabilities, and electronic properties of small bimetallic Be₂Au _n (n = 1–9) clusters. The optimized geometries reveal that the most stable isomers have 3D structures at n = 3, 5, 7, 8, and 9. Here, the relative stabilities were investigated in terms of the averaged atomic binding energies, fragmentation energies and second-order difference of energies. The results show that the planar Be₂Au₄ structure is the most stable structure for Be₂Au _n clusters. The HOMO−LUMO gap, vertical ionization potential, vertical electron affinity and chemical hardness exhibit a pronounced even–odd alternating phenomenon. In addition, charge transfer and natural electron configuration were analyzed and compared.     

Density functional theory study on (LiNH<Subscript>2</Subscript>)<Subscript>n</Subscript> (<Emphasis Type="Italic">n</Emphasis> = 1–5) clusters

Geometrical structures and relative stabilities of (LiNH₂)_n (n = 1–5) clusters were studied using density functional theory (DFT) at the B3LYP/6-31G* and B3LYP/6-31++G* levels. The electronic structures, vibrational properties, N–H bond dissociation energies (BDE), thermodynamic properties, bond properties and ionization potentials were analyzed for the most stable isomers. The calculated results show that the Li–N and Li–Li bonds can be formed more easily than those of the Li–H or N–H bonds in the clusters, in which NH₂ is bound to the framework of Li atomic clusters with fused rings. The average binding energies for each LiNH₂ unit increase gradually from 142 kJ mol⁻¹ up to about 180 kJ mol⁻¹ with increasing n. Natural bond orbital (NBO) analysis suggests that the bonds between Li and NH₂ are of strong ionicity. Three-center–two-electron Li–N–Li bonding exists in the (LiNH₂)₂ dimer. The N–H BDE values indicate that the change in N–H BDE values from the monomer a1 to the singlet-state clusters is small. The N–H bonds in singlet state clusters are stable, while the N–H bonds in triplet clusters dissociate easily. A study of their thermodynamic properties suggests that monomer a1 forms clusters (b1, c1, d2 and e1) easily at low temperature, and clusters with fewer numbers of rings tend to transfer to ones with more rings at low temperature. E _g, E _HOMO and E _av decrease gradually, and become constant. Ring-like (LiNH₂)_3,4 clusters possess higher ionization energy (VIE) and E _g, but lower values of E _HOMO. Ring-like (LiNH₂)_3,4 clusters are more stable than other types. A comparison of structures and spectra between clusters and crystal showed that the NH₂ moiety in clusters has a structure and spectral features similar to those of the crystal.