Photoinduced intramolecular charge-transfer reactions in 4-amino-3-methyl benzoic acid methyl ester: A fluorescence study in condensed-phase and jet-cooled molecular beams

Photoinduced intramolecular charge-transfer reactions in 4-amino-3-methyl benzoic acid methyl ester (AMBME) have been investigated spectroscopically. AMBME, with its weak charge donor primary amino group, shows dual emission in polar solvents. Absorption and emission measurements in the condensed phase support the premise that the short wavelength emission band corresponds to local emission and the long wavelength emission band to the charge transfer emission. Laser-induced fluorescence excitation spectra show the presence of two low-energy conformers in jet-cooled molecular beams. Theoretical calculations using density functional theory help to determine structure, vibrational modes, potential energy surface, transition energy and oscillator strength for correlating experimental findings with theoretical results.     

Photoinduced intramolecular charge transfer (ICT) reaction in trans-methyl p-(dimethylamino) cinnamate: A combined fluorescence measurement and quantum chemical calculations

The photophysical behaviour of trans-methyl p-(dimethylamino) cinnamate (t-MDMAC) donor–acceptor system has been investigated by steady-state absorption and emission spectroscopy and quantum chemical calculations. The molecule t-MDMAC shows an emission from the locally excited state in non-polar solvents. In addition to weak local emission, a strong solvent dependent red shifted fluorescence in polar aprotic solvents is attributed to highly polar intramolecular charge transfer state. However, the formation of hydrogen-bonded clusters with polar protic solvents has been suggested from a linear correlation between the observed red shifted fluorescence band maxima with hydrogen bonding parameters (). Calculations by ab initio and density functional theory show that the lone pair electron at nitrogen center is out of plane of the benzene ring in the global minimum ground state structure. In the gas phase, a potential energy surface along the twist coordinate at the donor (–NMe₂) and acceptor (–CH = CHCOOMe) sites shows stabilization of S₁ state and destabilization S₂ and S₀ states. A similar potential energy calculation along the twist coordinate in acetonitrile solvent using non-equilibrium polarized continuum model also shows more stabilization of S₁ state relative to other states and supports solvent dependent red shifted emission properties. In all types of calculations it is found that the nitrogen lone pair is delocalized over the benzene ring in the global minimum ground state and is localized on the nitrogen centre at the 90° twisted configuration. The S₁ energy state stabilization along the twist coordinate at the donor site and localized nitrogen lone pair at the perpendicular configuration support well the observed dual fluorescence in terms of proposed twisted intramolecular charge transfer (TICT) model.     

Solvatochromism of Heteroaromatic Compounds: VII. Bifurcate Hydrogen Bond in Solvatochromic Complexes of 2-(1,2,2-Tritricyanovinyl)pyrrole

IR and UV spectrosocopy and quantum chemistry were used to reveal a bifurcate (three-center) hydrogen bond in the complexes of 2-(1,2,2-tricyanovinyl)pyrrole with hydrogen-bond acceptors. In the gas phase and aprotic inert solvents this compound exists predominantly as the sp conformer stabilized by intra- molecular hydrogen bond involving an orthogonal system of the nitrile group and the NH hydrogen. The formation of the three-center hydrogen bond with an aprotic protophilic solvent shifts the conformational equilibrium to the ap conformer. As the strength of the intermolecular hydrogen bond increases in the series nitromethane < acetonitrile < dioxane, the fraction of the ap conformer increases and becomes prevailing already in the moderately protophilic THF. Solvatochromism of the long-wave absorption band in the elec- tronic spectra of the sp and ap conformers was studied in detail.     

The conformers of phenylglycine

The neutral form of the unnatural amino acid phenylglycine was vaporized by laser ablation, and the presence of two conformers was detected in a supersonic expansion by Fourier transform microwave spectroscopy. Both conformers were unequivocally identified by comparison of their experimental rotational and quadrupole coupling constants with those calculated ab initio. The most stable conformer is stabilized by intramolecular hydrogen bonds N-H...O=C, N-H...pi (with the closest C-C bond in the aromatic ring), and a cis-COOH interaction. The other conformer exhibits a O-H...N hydrogen bond between the hydrogen atom of the hydroxyl group and the lone pair at the nitrogen atom.     

Ab initio study of anomeric effect in 2,2-difluoroglycine

The optimized molecular structures of seven conformations of 2,2-difluoroglycine have been obtained from ab initio calculations. For conformers in which the lone pair of electrons on the nitrogen are antiperiplanar to one of the C–F bonds, that C–F bond is longer than the other C–F bond, which is synperiplanar to the lone pair of electrons. Conformers which have these features are the most stable conformers of those examined. This observation is explained in terms of an anomeric effect of the 1p(N)→σ^*(C–F). At the MP2/6-31G^* level of calculation, conformers IV and V are 21.5 and 18.7 kJ/mol, respectively, more stable than the least stable conformer, VI, which does not exhibit an anomeric effect. Conformer VII was found to be exceptionally stable, in addition to an anomeric effect, this conformer also exhibits features of a FH–O hydrogen bond.     

Photophysical Properties of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine

Spectral and photophysical investigations of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine (APT) have been performed in various solvents with different polarity and hydrogen-bonding ability.The emission spectra of APT are found to exhibit dual fluorescence in polar solvents, which attributes to the local excited and intramolecular charge transfer states, respectively. The two-state model is proven out for APT in polar solvent by the time-correlated single photon counting emission decay measurement. Interestingly, the linear relationships of different emission maxima and solvent polarity parameter are found for APT in protic and aprotic solvents, because of the hydrogen bond formation between APT and alcohols at the amino nitrogen N25. Furthermore, the effects of the complexation of the metal ion with tpy group of APT and the hydrogen bond formation between APT with methanol at the terpyridinenitrogen N4—N8—N14 are also presented. The appearance of new long-wave absorption and fluorescence bands indicates that a new ground state of the complexes is formed.     

Shape of 4S- and 4R-hydroxyproline in gas phase

The alpha-amino acids 4(S)-hydroxyproline and 4(R)-hydroxyproline have been studied under isolation conditions in gas phase using laser-ablation molecular-beam Fourier transform microwave spectroscopy. Two conformers of each molecule have been detected in the jet-cooled rotational spectrum. The most stable conformer in both molecules exhibits an intramolecular N...H-O hydrogen bond (configuration 1) between the hydrogen atom of the carboxylic group and the nitrogen atom. The second conformer is characterized by an intramolecular N-H...O=C hydrogen bond (configuration 2). The conformers of 4(R)-hydroxyproline adopt a C(gamma)-exo puckering, while those of 4(S)-hydroxyproline present a C(gamma)-endo ring conformation. These ring conformations, which show the same propensity observed in collagen-like peptides, are stabilized by additional intramolecular hydrogen bonds involving the 4-hydroxyl group, with the exception of the most stable form of 4(S)-hydroxyproline for which a n-pi interaction between the oxygen atom of the 4-hydroxyl group and the carboxyl group carbon seems to be established. A gauche effect could be also contributing to stabilize the observed conformers.     

Gas-phase structure of N,N-dimethylglycine.

Three conformers of the neutral amino acid N,N-dimethylglycine [(CH3)2NCH2COOH] were detected in a supersonic expansion by a combination of laser ablation (LA) and molecular-beam Fourier transform microwave (MB-FTMW) spectroscopy. A bifurcated methyl-to-carbonyl (C--HO==C) weak intramolecular hydrogen bond might stabilise the most stable conformer of C(s) symmetry. The second most stable conformer of C1 symmetry has a hydrogen bond between the hydroxyl group and the lone pair at the nitrogen atom (NH--O). The r(s) and r0 structures were derived for this conformer from the rotational data for the parent and six minor 13C, 15N and OD isotopomers. A third conformer exhibits a cis-carboxyl functional group and C1 symmetry. Ab initio MP2/6-311++G(d,p) predictions of the spectroscopic parameters were useful in analysing the spectra. In particular, the agreement of the nuclear quadrupole coupling constants with those calculated was conclusive in identifying the different conformers.     

Theoretical analysis of fluoroglycine conformers

Seven different optimized conformers of α‐fluoroglycine (H₂NCHFCOOH) were obtained from ab initio calculations. Some of these conformers are exceptionally stable compared to similar conformers of glycine. Conformers in which the lone pair of electrons on the nitrogen atom are antiperiplanar to the C F bond are more stable than conformers that do not have such an arrangement. The stability difference between conformers with such an arrangement and conformers that have the lone pair of electrons synperiplanar to the C F bond is about 27 kJ/mol (calculated at the MP2/6‐31+G* level). Conformers that have the lone pair of electrons antiperiplanar to the C F bond possess a longer C F bond, a shorter C N bond, and sp²‐like amino bond angles. For some conformers an unusual hydrogen bond involving the acidic carboxylic acid hydrogen and the electronegative fluorine atom is observed. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 426–431, 2000     

Unusual conformational effect in alpha-aminoorganostannanes

[formula: see text] Dynamic NMR analysis of conformationally mobile and rigid 2-tributylstannyl-N-methylpiperidines revealed an unexpected conformational effect that is manifested in a small energy difference between conformers in which the tin is equatorial and axial. The major reason appears to be a distortion of the conformer in which the C-2-Sn bond is synclinal to the nitrogen lone pair.     

Benzyl alcohol-ammonia (1:1) cluster structure investigated by combined IR-UV double resonance spectroscopy in jet and ab initio calculation

Laser-induced fluorescence excitation and IR-UV double resonance spectroscopy have been used to determine the hydrogen-bonded structure of benzyl alcohol-ammonia (1:1) cluster in a jet-cooled molecular beam. In addition,ab initio quantum chemical calculations have been performed at HF/6-31G and HF/6-31G(d,p) levels for different ground state equilibrium structures of the cluster to correlate the calculated OH and NH frequencies and their intensities with experimental results. The broad red-shifted OH-stretching mode in the IR-UV double resonance spectrum suggests strong hydrogen bonding between the hydroxyl hydrogen and the lone pair of the ammonia nitrogen. The position and intensity distribution of the calculated NH and OH modes for the minimum-energy gauche form at HF/6-31G level have better correlation with the experimental results compared to other calculated ground state equilibrium conformers. These results lead to the conclusion that the minimum energy gauche form of the cluster is populated in the jet-cooled condition.     

Tautomer selective photochemistry in 1-(tetrazol-5-yl)ethanol

A combined matrix isolation FTIR and theoretical DFT/B3LYP/6-311++G(d,p) study of the molecular structure and photochemistry of 1-(tetrazol-5-yl)ethanol [1-TE] was performed. The potential energy surface landscapes of the 1H and 2H tautomers of the compound were investigated and the theoretical results were used to help characterize the conformational mixture existing in equilibrium in the gas phase prior to deposition of the matrices, as well as the conformers trapped in the latter. In the gas phase, at room temperature, the compound exists as a mixture of 12 conformers (five of the 1H tautomer and seven of the 2H tautomer). Upon deposition of the compound in an argon matrix at 10 K, only three main forms survive, because the low barriers for conformational isomerization allow extensive conformational cooling during deposition. Deposition of the matrix at 30 K led to further simplification of the conformational mixture with only one conformer of each tautomer of 1-TE surviving. These conformers correspond to the most stable forms of each tautomer, which bear different types of intramolecular H-bonds: 1H-I has an NH···O hydrogen bond, whereas 2H-I has an OH···N hydrogen bond. Upon irradiating with UV light (λ > 200 nm), a matrix containing both 1H-I and 2H-I forms, an unprecedented tautomer selective photochemistry was observed, with the 2H tautomeric form undergoing unimolecular decomposition to azide + hydroxypropanenitrile and the 1H-tautomer being photostable.     

Solvent dependent excited state spectral properties of 4-hydroxyacridine: Evidence for only water mediated excited state proton transfer process

The possibility of ground and excited state proton transfer reaction across the five member intramolecular hydrogen bonded ring in 4-hydroxyacridine (4-HA) has been investigated spectroscopically and the experimental results have been correlated with quantum chemical calculations. The difference in the emissive behaviour of 4-HA in different types of solvents is due to the presence of different species in the excited state. In non-polar solvents, the species present is non-fluorescing in nature, whereas 4-HA molecule shows normal emission from intramolecularly hydrogen bonded closed conformer in polar aprotic solvents. In polar protic solvents like MeOH, EtOH, etc. (except water), a single broad emission band is attributed to the hydrogen bonded solvated form of 4-HA. However, in case of water, fluorescence from the tautomeric form of 4-HA is observed apart from emission from the solvated form. Emission from the tautomeric form may arise due to double proton transfer via a single water molecule bonded to 4-HA. Evaluation of the potential energy surfaces by quantum chemical calculations using density functional theory (DFT) and time dependent density functional theory (TDDFT), however, points towards the possibility of proton transfer—both intrinsic intramolecular as well as water mediated in the first excited state of 4-HA.     

The singular gas-phase structure of 1-aminocyclopropanecarboxylic acid (Ac3c)

The natural nonproteinogenic α-amino acid 1-aminocyclopropanecarboxylic acid (Ac(3)c) has been vaporized by laser ablation and studied in the gas phase by molecular-beam Fourier transform microwave spectroscopy. Comparison of the experimental rotational and (14)N nuclear quadrupole coupling constants with the values predicted ab initio for these parameters has allowed the unambiguous identification of three Ac(3)c conformers differing in the hydrogen bonding pattern. Two of them resemble those characterized before for the coded aliphatic α-amino acids. Remarkably, a third conformer predicted to be energetically accessible for all of these amino acids but never observed (the so-called "missing conformer") has been found for Ac(3)c, close in energy to the global minimum. This is the first time that such a conformer, stabilized by an N-H···O(H) hydrogen bond, is detected in the rotational spectrum of a gaseous α-amino acid with a nonpolar side chain. The conjugative interaction established between the cyclopropane ring and the adjacent carbonyl group seems to be responsible for the unique conformational properties exhibited by Ac(3)c.     

Electronic structure and conformational properties of (carboxy-alkenyl)-phosphonic acids

The general conformational properties and electronic structure of (carboxy-alkenyl)-phosphonic derivatives were determined at RHF/STO-3G^* level. In all the series, low rotation barriers were found for the two C=C/P=O conformers. In the compounds in which the interactions between the carboxylic and phosphonic moieties are smaller, the most stable conformers are the C=C/P=O s-cis ones. In most of the conformers, the C=C/C=O system presents the disposition s-cis. The Z-(2-carboxy-vinyl) and Z-(2-carboxy-propenyl) phosphonic acids present intramolecular hydrogen bonds, existing in at least four conformer with internal hydrogen bonds. These last compounds were more rigorously studied at RHF/3-21G^* and RHF/6-31G^** levels. The most stable conformer shows a trans structure for the C=C/P=O angle, with an intramolecular hydrogen bond located between the hydroxylic hydrogen of phosphonic group and the carbonyl oxygen of carboxylic moiety. A secondary conformer is found with a double intramolecular hydrogen bond between two hydroxylic hydrogens of the phosphonic moiety and the oxygen of carboxylic bond. Another secondary conformer appears with an intramolecular hydrogen bond between the oxygen of the phosphoryl bond and the hydroxylic hydrogen of the carboxylic group. A study of the topology of charge densities is carried out. This analysis reveals bonds with an ionic participation. A very weak π conjugation, variable with the conformers, is found in the C=C/P=O system, as well as a strongly polarized P=O partial triple bond. The intramolecular hydrogen bonds give rise to cyclic structures.     

(CH3)2S与HOCl分子间的卤键和氢键相互作用

在DFT-B3LYP/6-311++G**水平上分别求得(CH3)2S…ClOH卤键复合物和(CH3)2S…HOCl氢键复合物势能面上的稳定构型. 频率分析表明, 与单体HOCl相比, 在两种复合物中, 10Cl—11O和12H—11O键伸缩振动频率发生显著的红移. 经MP2/6-311++G**水平计算的含基组重叠误差(BSSE)校正的气相中相互作用能分别为-11.69和-24.16 kJ·mol-1. 自然键轨道理论(NBO)分析表明, 在(CH3)2S…ClOH卤键复合物中, 引起10Cl—11O键变长的因素包括两种电荷转移: (i) 孤对电子LP(1S)1→σ*(10Cl—11O); (ii) 孤对电子LP(1S)2→σ*(10Cl—11O), 其中孤对电子LP(1S)2→σ*(10Cl—11O)转移占主要作用, 总的结果是使σ*(10Cl—11O)的自然布居数增加0.14035e, 同时11O原子的再杂化使其与10Cl成键时s成分增加, 即具有与电荷转移作用同样的“拉长效应”; 在(CH3)2S…HOCl氢键复合物中也存在类似的电荷转移, 但是11O原子的再杂化不同于前者. 自然键共振理论(NRT)进行键序分析表明, 在卤键复合物和氢键复合物中, 10Cl—11O和12H—11O键的键序都减小. 通过分子中原子理论(AIM)分析了复合物中卤键和氢键的电子密度拓扑性质.     

Impact of azaproline on Peptide conformation

The amino acid analog azaproline (azPro) contains a nitrogen atom in place of the C(alpha) of proline. Peptides containing azPro were shown to stabilize the cis-amide conformer for the acyl-azPro bond and prefer type VI beta-turns both in crystals and in organic solvents by NMR. The increased stability for cis-amide conformers was relatively minor with respect to the trans-conformers. Further, their conformational preferences were depended on solvent. To elucidate the impact of azPro substitution on amide cis-trans isomerism and peptide conformation, this paper reports ab initio studies on azPro derivatives and a comparison with their cognate Pro derivatives: 1-acetyl-2-methyl pyrrolidine (1), 1-acetyl-2-methyl pyrazolidine (2), Ac-Pro-NHMe (3), Ac-azPro-NHMe (4), Ac-azPro-NMe(2) (5), Ac-azAzc-NHMe (6), and Ac-azPip-NHMe (7). Conformational preferences were explored at the MP2/6-31+G** level of theory in vacuo. Solvation effects for 1 and 2 were studied implicitly using the polarizable continuum model and explicitly represented by interactions with a single water molecule. An increase in the conformational preference for the cis-amide conformer of azPro was clearly seen. An intramolecular hydrogen bond occurred solely in the trans-amide conformer that reduced the preference for the cis-conformer by 2.2 kcal/mol. The larger ring homolog aza-pipecolic acid (azPip), in which this internal hydrogen bond was diminished, significantly augmented stabilization of the cis-amide conformer. In aqueous solution, the preference for the cis-amide conformers was greatly reduced, mainly as a result of interaction between water and the lone pair of the alpha-nitrogen in the trans-amide conformer that was 3.8 kcal/mol greater than that in the cis-conformer. In the azPro analog, the energy barrier for cis-trans amide isomerization was 6 kcal/mol less than that in the cognate Pro derivative. Because the azPro derivatives can stabilize the cis-amide bond and mimic a type VI beta-turn without incorporation of additional steric bulk, such a simple chemical modification of the peptide backbone provides a useful conformational constraint when incorporated into the structure of selected bioactive peptides. Such modifications can scan receptors for biological recognition of reverse turns containing cis-amide bonds by the incorporation of type VI beta-turn scaffolds with oriented appended side chains.     

Electronic and infrared spectroscopy of jet-cooled (+/-)-cis-1-amino-indan-2-ol hydrates

The role of conformational isomerism in molecular interaction has been studied using the example of jet-cooled complexes of (+/-)-cis-1-amino-indan-2-ol with water. The two formerly evidenced conformers of (+/-)-cis-1-amino-indan-2-ol easily form hydrates and dihydrates, which have been studied by means of laser-induced fluorescence and IR/UV double resonance spectroscopy, as well as ab initio calculations. All the 1 : 1 and 1 : 2 complexes with water evidenced in this work involve "ring" structures, in which the water monomer or dimer acts as an acceptor from the NH(2) and a donor to the OH groups of (+/-)-cis-1-amino-indan-2-ol. However, the water lies externally to the indan frame in the hydrates of conformer I of (+/-)-cis-1-amino-indan-2-ol, which possesses axial NH(2) and equatorial OH groups, and above it for the hydrates with the less stable conformer II, with equatorial NH(2) and axial OH groups. Consequently, the different steric constraints which exist in the two conformers result in different hydrogen bond topologies, with an additional OH[dot dot dot]pi interaction for the hydrates of conformer II.     

Vibrational studies of sulfamoil chloride (ClSO2NH2)

The infrared spectrum of sulfamoil chloride in the liquid phase was reinvestigated; the infrared and Raman spectra of the solid phase have also been obtained. A complete assignment of the observed bands is proposed. A subsequent normal coordinate analysis was performed. The experimental data are compared to results of ab initio and DFT (density functional theory) calculations. According to the experimental and theoretical results the main conformer of ClSO₂NH₂ possesses an anti conformation (C_s symmetry, S---Cl single bond in anti position with respect to the nitrogen lone pair).     

Photophysical behavior of trans-2-styrylquinoxaline conformers

The fluorescence and excitation spectra of trans-2-styrylquinoxaline changed markedly on varying the excitation and emission wavelengths, probably because of an equilibrium between conformers originating from the rotation of a quasi-single bond between the quinoxalyl group and the ethylenic carbon atom. The values of the fluorescence quantum yields depend on the solvent polarity at room temperature. However, the fluorescence of each conformer shows a different dependence on the solvent polarity and only one conformer is fluorescent in non-polar hydrocarbon solvents, while the composition of the two conformers does not change significantly on varying the solvent.