IR-UV double resonance spectroscopic investigation of phenylacetylene-alcohol complexes. Alkyl group induced hydrogen bond switching

The electronic transitions of phenylacetylene complexes with water and trifluoroethanol are shifted to the blue, while the corresponding transitions for methanol and ethanol complexes are shifted to the red relative to the phenylacetylene monomer. Fluorescence dip infrared (FDIR) spectra in the O-H stretching region indicate that, in all the cases, phenylacetylene is acting as a hydrogen bond acceptor to the alcohols. The FDIR spectrum in the acetylenic C-H stretching region shows Fermi resonance bands for the bare phenylacetylene, which act as a sensitive tool to probe the intermolecular structures. The FDIR spectra reveal that water and trifluoroethanol interact with the pi electron density of the acetylene C-C triple bond, while methanol and ethanol interact with the pi electron density of the benzene ring. It can be inferred that the hydrogen bonding acceptor site on phenylacetylene switches from the acetylene pi to the benzene pi with lowering in the partial charge on the hydrogen atom of the OH group. The most significant finding is that the intermolecular structures of water and methanol complexes are notably distinct, which, to the best of our knowledge, this is first such observation in the case of complexes of substituted benzenes.     

Structure of the phenylacetylene-water complex as revealed by infrared-ultraviolet double resonance spectroscopy

The structure of the phenylacetylene-water complex has been elucidated based on spectral shifts in electronic and vibrational transitions. Phenylacetylene forms a cyclic complex with water incorporating C-H...O and O-H...pi hydrogen bonds, which is different from both the benzene-water and acetylene-water complexes, even though phenylacetylene combines the features of both benzene and acetylene. Formation of such a complex can be rationalized on the basis of cooperativity between the two sets of hydrogen bonds.     

Transformations of griseofulvin in strong acidic conditions--crystal structures of 2'-demethylgriseofulvin and dimerized griseofulvin

The structure of griseofulvic acid, C16H15ClO6, at 100 K has orthorhombic (P2(1)2(1)2) symmetry. It is of interest with respect to biological activity. The structure displays intermolecular O-H...O, C-H...O hydrogen bonding as well as week C-H...pi and pi...pi interactions. In strong acidic conditions the griseofulvin undergoes dimerization. The structure of dimerized griseofulvin, C34H32C12O12 x C2H6O x H2O, at 100 K has monoclinic (P2(1)) symmetry. The molecule crystallized as a solvate with one ethanol and one water molecule. The dimeric molecules form intermolecular O-H...O hydrogen bonds to solvents molecules only but they interact via week C-H...O, C-H...pi, C-Cl...pi and pi...pi interactions with other dimerized molecules.     

Solvation of H3O+ by phenol: hydrogen bonding vs pi complexation

Ab initio calculations at the second-order M?ller-Plesset perturbation theoretic level have been carried out to study the solvation of protonated water by phenol molecules. The results show that in addition to classical O-H...O hydrogen bonds, C-H...O, pi...H-O, and pi...H-C bonds are also formed, thus stabilizing the H3O+(C(6)H(5)OH)3 complex.     

Complexes of atmospheric alpha-dicarbonyls with water: FTIR matrix isolation and theoretical study

The complexes of glyoxal (Gly), methylglyoxal (MGly), and diacetyl (DAc) with water have been studied using Fourier transform infrared (FTIR) matrix isolation spectroscopy and MP2 calculations with 6-311++G(2d,2p) basis set. The analysis of the experimental spectra of the Gly(MGly,DAc)/H2O/Ar matrixes indicates formation of one Gly...H2O complex, three MGly...H2O complexes, and two DAc...H2O ones. All the complexes are stabilized by the O-H...O(C) hydrogen bond between the water molecule and carbonyl oxygen as evidenced by the strong perturbation of the O-H, C=O stretching vibrations. The blue shift of the CH stretching vibration in the Gly...H2O complex and in two MGly...H2O ones suggests that these complexes are additionally stabilized by the improper C-H...O(H2) hydrogen bonding. The theoretical calculations confirm the experimental findings. They evidence the stability of three hydrogen-bonded Gly...H2O and DAc...H2O complexes and six MGly...H2O ones stabilized by the O-H...O(C) hydrogen bond. The calculated vibrational frequencies and geometrical parameters indicate that one DAc..H2O complexes, two Gly...H2O, and three MGly...H2O ones are additionally stabilized by the improper hydrogen bonding between the C-H group and water oxygen. The comparison of the theoretical frequencies with the experimental ones allowed us to attribute the calculated structures to the complexes present in the matrixes.     

Modeling of hydrogen bonds in monohydrated 2,4-dithiothymine: an ab initio and AIM study

Twelve tautomers of 2,4-dithiothymine are calculated at the MP2/6-31+G(d) level, and the most stable one is referred to the di-keto form (P12). Then four H-bonded complexes between P12 and water are optimized at the MP2/6-31+G(d) level of theory. The calculation of vibrational frequencies and natural bond orbital analysis are also carried out at the same level to investigate the hydrogen bonds involved in all the systems. Within all the four complexes, three types of hydrogen bonds are formed, in which the O-H...S and N-H...O bonds are the normal bonds with the X-H bond elongation and red shift of the corresponding stretch frequencies, while the C-H...O interaction is an improper, blue-shifting hydrogen bond accompanied with the contraction of the C-H bond and a blue shift of the C-H stretch frequency. The topological properties are investigated with the atoms-in-molecules (AIM) theory. The NMR chemical shielding for the isolated and the four monohydrated 2,4-dithiothymine are calculated using the "gauge-including atomic orbital" (GIAO) method. The 1H chemical shifts are influenced by the formation of hydrogen bonds.     

Theoretical investigation of in-plane hydrogen-bonded complexes of ammonia with partially substituted fluorobenzenes

Systematic investigation of in-plane hydrogen-bonded complexes of ammonia with partially substituted fluorobenzenes has revealed that fluorobenzene, difluorobenzene, and trifluorobenzene favor formation of cyclic complexes with a C-H...N-H...F-C binding motif. On the other hand, tetrafluorobenzene and pentafluorobenzene favor formation of linear C-H...N hydrogen-bonded complexes. The complete absence of exclusively linear N-H...F hydrogen-bonded complexes for the entire series indicates that C-F bond in fluorobenzenes is a reluctant hydrogen-bond acceptor. However, fluorine does hydrogen bond when cooperatively stabilized with C-H...N hydrogen bonds for the lower fluoro analogues. The propensity of fluorobenzenes to adapt to the C-H...N-H...F-C binding motif decreases with the progressive fluorination of the benzene ring and disappears completely when benzene ring is substituted with five or more fluorine atoms.     

Interaction of alcohols with 2-fluoro- and 4-fluorophenylacetylenes: infrared-optical double resonance spectroscopic and computational investigation

Alcohol complexes of 4-fluorophenylacetylene and 2-fluorophenylacetylene were investigated using IR-UV double resonance spectroscopy. Methanol forms a cyclic complex with both the fluorophenylacetylenes incorporating C-H···O and O-H···π hydrogen bonds, the structure of which is similar to that of the corresponding water complex but different from that of a phenylacetylene-methanol complex. The anti conformer of ethanol also binds in a similar fashion to both the fluorophenylacetylenes. Additionally, the gauche conformer of ethanol binds to 2-fluorophenylacetylene in a distinctly different structural motif that incorporates C-H···F and O-H···π hydrogen bonds. The OH group of trifluoroethanol interacts primarily with the π electron density of the C≡C bond. The π electron density of the C≡C bond is the principal point of interaction between the alcohols and both the fluorophenylacetylenes. The present results are indicative of the fact that fluorine substitution on the phenyl ring is sufficient to eliminate the subtle hydrogen bonding behavior of phenylacetylene.     

Spectral tuning by switching C-H[...]O hydrogen bonds: rotation-induced spectral shifts of 7-hydroxyquinoline HCOOH isomers

Spectral tuning effects on visible chromophores by hydrogen bonds are central to the chemistry of vision and of photosynthesis. A model for large spectral tuning effects by hydrogen bond switching is provided by the 7-hydroxyquinoline x HCOOH complex, which forms two isomers, CTN1 and CTN2, both with an HCOOH[...]N hydrogen bond but with different (quinoline)C-H[...]O=C hydrogen bonds. A 180 degrees rotation of the HCOOH moiety around the O-H[...]N hydrogen bond exchanges the C-H[...]O hydrogen bonds, rotates the dipole moment of HCOOH, and leads to an approximately 850 cm(-1) shift of the electronic spectrum. Mass-selected S1<--S0 resonant two-photon ionization, UV-UV holeburning, S1-->S0 fluorescence spectra, and photoionization efficiency curves of the two 7-hydroxyquinoline x HCOOH isomers were measured in supersonic expansions. Comparison to ab initio calculations allow us to determine the H-bond connectivity and structure of the two isomers and to assign their inter- and intramolecular vibrations. The Franck-Condon factors of the intermolecular shear vibration chi in the S1<--S0 spectra indicate that the weak C-H[...]O hydrogen bond contracts markedly in the CTN1 isomer but expands in the CTN2 isomer. These changes of H-bond lengths agree with the spectral shifts. In contrast, the strong O-H[...]N hydrogen bond undergoes little change upon S1[...]S0 excitation.     

Blue-shifted A-H stretching modes and cooperative hydrogen bonding. 1. Complexes of substituted formaldehyde with cyclic hydrogen fluoride and water clusters

The structures and vibrational spectra of the intermolecular complexes formed by insertion of substituted formaldehyde molecules HRCO (R = H, Li, F, Cl) into cyclic hydrogen fluoride and water clusters are studied at the MP2/aug-cc-pVTZ computational level. Depending on the nature of the substituent R, the cluster type, and its size, the C-H stretching modes of HRCO undergo large blue and partly red shifts, whereas all the F-H and O-H stretching modes of the conventional hydrogen bonds are strongly red-shifted. It is shown that (i) the mechanism of blue shifting can be explained within the concept of the negative intramolecular coupling between C-H and C=O bonds that is inherent to the HRCO monomers, (ii) the blue shifts also occur even if no hydrogen bond is formed, and (iii) variation of the acceptor X or the strength of the C-H...X hydrogen bond may either amplify the blue shift or cause a transition from blue shift to red shift. These findings are illustrated by means of intra- and intermolecular scans of the potential energy surfaces. The performance of the negative intramolecular coupling between C-H and C=O bonds of H(2)CO is interpreted in terms of the NBO analysis of the isolated H(2)CO molecule and H(2)CO interacting with (H2O)n and (HF)n clusters.     

Geometric isotope effect of various intermolecular and intramolecular C-H...O hydrogen bonds, using the multicomponent molecular orbital method

The geometric isotope effect (GIE) of sp- (acetylene-water), sp(2)- (ethylene-water), and sp(3)- (methane-water) hybridized intermolecular C-H...O and C-D...O hydrogen bonds has been analyzed at the HF/6-31++G level by using the multicomponent molecular orbital method, which directly takes account of the quantum effect of proton/deuteron. In the acetylene-water case, the elongation of C-H length due to the formation of the hydrogen bond is found to be greater than that of C-D. In contrast to sp-type, the contraction of C-H length in methane-water is smaller than that of C-D. After the formation of hydrogen bonds, the C-H length itself in all complexes is longer than C-D and the H...O distance is shorter than D...O, similar to the GIE of conventional hydrogen bonds. Furthermore, the exponent (alpha) value is decreased with the formation of the hydrogen bond, which indicates the stabilization of intermolecular C-H...O hydrogen bonds as well as conventional hydrogen bonds. In addition, the geometric difference induced by the H/D isotope effect of the intramolecular C-H...O hydrogen bond shows the same tendency as that of intermolecular C-H...O. Our study clearly demonstrates that C-H...O hydrogen bonds can be categorized as typical hydrogen bonds from the viewpoint of GIE, irrespective of the hybridizing state of carbon and inter- or intramolecular hydrogen bond.     

Study of the interaction in clusters formed by phenol and CH3X (X=CN,F,Cl) molecules

The characteristics of the interaction between phenol and acetonitrile, methyl fluoride and methyl chloride were studied. The most stable structures for clusters containing one or two CH3X molecules and one phenol moiety were located by means of ab initio and density functional theory calculations. Phenol-acetonitrile dimer presents two almost equally stable structures; one of them is a typical linearly hydrogen bonded minimum, whereas in the other one, a C-H...pi contact is established accompanied by a distorted O-H...N hydrogen bond. Although the latter minimum presents the larger interaction energy, deformation effects favor the formation of the linear hydrogen bonded one. In complexes with methyl fluoride and methyl chloride, this arrangement is the most stable structure and no linear hydrogen bonded structures were located. Our best estimates for the interaction energies amount to -27.8, -21.6, and -19.7 kJ/mol for clusters of phenol with acetonitrile, methyl fluoride, and methyl chloride, respectively. The main contribution to the stabilization of these clusters is of electrostatic nature, although in structures where a C-H...pi contact is present, the dispersion contribution is also significant. In clusters formed by phenol and two CH3X units, the most stable arrangement corresponds to a head to tail disposal with O-H...X, C-H...X, and C-H...pi contacts forming a cycle. Only for this type of arrangement, three body effects are non-negligible even though they constitute a minor effect. The results also indicate that interactions with methyl fluoride and methyl chloride are of similar intensity, although weaker than with acetonitrile. Significant frequency shifts are predicted for the O-H stretching, which increase when increasing the number of CH3X molecules.     

Carboxylic acid clathrate hosts of Diels-Alder adducts of phencyclone and 2-alkenoic acids. Role of bidentate C-H ... O hydrogen bonds between the phenanthrene and carbonyl groups in host-host networks

Carboxylic acid host compounds (3) having a phenanthrene-condensed bicyclo[2.2.1]hept-2-en-7-one skeleton have been synthesized by the [4 + 2]pi cycloaddition of phencyclone (1a) with 2-alkenoic acids (2) and their inclusion behavior was investigated. The endo [4 + 2]pi cycloadducts (3) enclathrated alcohols and ethers besides aromatics and ketones. The X-ray crystallographic analysis of the inclusion compound (3ac-dioxane) of the endo [4 + 2]pi cycloadduct (3ac) of phencyclone and trans 2-butenoic acid (2c) indicated that dioxanes are located at the opposite side of the bridged carbonyl of the bicyclo[2.2.1]hept-2-en-7-one moiety, in which the O-H...O and C-H...O hydrogen bonds play an important role in the inclusion complex formation. Similarly, a pair of 3-pentanone molecules were included in the endo [4 + 2] pi cycloadduct (3ae) of 1a and cinnamic acid (2e). In both cases, the hosts are linked by the edge-to-face interaction between the phenanthrene and phenyl rings and the "bidentate" C-H...O hydrogen bonds between the phenanthrene-ring hydrogens and the bridged carbonyl or the carboxylic carbonyl group. The endo [4 + 2] pi cycloadduct (3bl) of tetracyclone (1b) and acrylamide (2l) also showed a wide-range inclusion behavior, in which alcohols are included by making a hydrogen-bond loop with the amide groups. The inclusion behavior of the carboxylic acid Diels-Alder hosts is discussed on the basis of the single crystal X-ray analysis, thermal analysis and semiempirical molecular orbital calculation data.     

Evidence for a C-H...pi type weak interaction: 1 : 1 complex of styrene with acetylene studied by mass selective high-resolution UV spectroscopy and ab initio calculations

The 1 : 1 complex of styrene with acetylene has been studied by mass selective low- and high-resolution UV resonance-enhanced two-photon ionisation (R2PI) spectroscopy combined with genetic-algorithm-based computer-aided fit of the spectra with partial rotational resolution, and high level ab initio quantum chemistry calculations. Two stable conformeric geometries of the 1 : 1 complex of styrene and acetylene have been theoretically found: one with acetylene binding to styrene as a proton donor, and one with acetylene acting as a proton acceptor. From the analysis of the vibronic structure of the S1<-- S0 spectrum and the fit of the highly resolved spectrum of the 0 origin band of the complex it is shown that the favoured conformation is the one in which acetylene binds to the benzene ring of styrene through formation of a non-conventional hydrogen bond of C-H...pi type with no marked change of the transition moment orientation of styrene. The styrene moiety remains planar and the acetylene molecule is tilted by a small angle of 4 degrees relative to the C6 symmetry axis of the benzene ring, most likely due to the reduced symmetry of the benzene ring pi electrons rather than to a direct interaction with the vinyl group.     

Infrared-optical double resonance spectroscopic investigation of trifluoromethylphenols and their water complexes

The hydrogen bonding behavior of trifluoromethylphenols and their water complexes were investigated using IR-UV double resonance spectroscopy. Both ortho- and meta-trifluoromethylphenols exist in the syn conformer, which is the global minimum in both the cases. The IR spectrum in the O-H stretching region reveals the absence of an intramolecular O-H···F hydrogen bond in the syn-o-trifluoromethylphenol, which is in contrast to the results reported in the literature. The water complexes of both o-trifluoromethylphenol and m-trifluoromethylphenol are characterized by formation of O-H···O hydrogen bonds between the donor phenolic OH group and the acceptor water molecule. In addition, the o-trifluoromethylphenol-(water)(2) complex was also observed.     

Recurrence of carboxylic acid-pyridine supramolecular synthon in the crystal structures of some pyrazinecarboxylic acids.

X-ray crystal structures of pyrazinic acid 1 and isomeric methylpyrazine carboxylic acids 2-4 are analyzed to examine the occurrence of carboxylic acid-pyridine supramolecular synthon V in these heterocyclic acids. Synthon V, assembled by (carboxyl)O-H...N(pyridine) and (pyridine)C-H...O(carbonyl) hydrogen bonds, controls self-assembly in the crystal structures of pyridine and pyrazine monocarboxylic acids. The recurrence of acid-pyridine heterodimer V compared to the more common acid-acid homodimer I in the crystal structures of pyridine and pyrazine monocarboxylic acids is explained by energy computations in the RHF 6-31G* basis set. Both the O-H.N and the C-H...O hydrogen bonds in synthon V result from activated acidic donor and basic acceptor atoms in 1-4. Pyrazine 2,3- and 2,5-dicarboxylic acids 10 and 11 crystallize as dihydrates with a (carboxyl)O-H...O(water) hydrogen bond in synthon VII, a recurring pattern in the diacid structures. In summary, the carboxylic acid group forms an O-H...N hydrogen bond in pyrazine monocarboxylic acids and an O-H...O hydrogen bond in pyrazine dicarboxylic acids. This structural analysis correlates molecular features with supramolecular synthons in pyridine and pyrazine carboxylic acids for future crystal engineering strategies.     

Cocrystallization of N-donor type compounds with 5-sulfosalicylic acid: The effect of hydrogen-bonding supramolecular architectures

Benzotriazole,N,N’-dimethylpiperazine and N-methylpiperazine were applied to crystallize with 5-sulfosalicylic acid(5-H2SSA),affording three new binary molecular cocrystals [(C6H6N3+).(C7H5O6S-)].H2O(1),[(C6H16N22+)1/2.(C7H5O6S-)].H2O(2) and [(C5H14N22+).(C7H5O6S-)2].3H2O(3) under general conditions.Proton-transferring occurs from acid to nitrogen of N-donor compounds in all compounds 1,2 and 3.Analysis of the hydrogen-bonding synthons and their effects on crystal packing were also presented in the context of crystal engineering and host-guest chemistry.In compound 1,1-D infinite chains are extended to a 2-D layered architecture via strong O-H...O hydrogen bonds and then to a 3-D network by N-H...O interactions.Compound 2 and 3 both have the 1-D chain which is formed by O-H...O bonds and weak C-H...O hydrogen bonds.A common intramolecular S(6) [synthon I] ring is formed by the hydroxyl with the carboxyl group in all three compounds.     

pi-H...O hydrogen bonds: multicenter covalent pi-H interaction acts as the proton-donating system

The MP2 method and the Pople-style basis sets 6-311++G(d,p), 6-311++G(2df,2pd), and 6-311++G(3df,3pd) were used to perform calculations on H3O+...C2H2 and C2H3+...C2H2 complexes and related species. Hydrogen bonds existing for the analyzed complexes were investigated as well as related pi-H...O --> pi...H-O and pi-H...pi --> pi...H-pi proton-transfer processes. For some of the complexes analyzed the multicenter pi-H interaction possessing the properties of a covalent bond acts as a proton donor; more generally it is classified as the Lewis acid. The quantum theory of "atoms in molecules" (QTAIM) was also applied to deepen the nature of these interactions in terms of characteristics of bond critical points. The pi-H...O, O-H...pi, and pi-H...pi interactions analyzed here may be classified as hydrogen bonds since their characteristics are the same as or at least similar to those of typical hydrogen bonds. H...pi interactions are common in crystal structures of organic and organometallic compounds. The analyses performed here show a continuum of such interactions since there are H...pi contacts possessing the characteristics of weak intermolecular interactions on the one hand and pi-H multicenter covalent bonds on the other. Ab initio and QTAIM results support the latter statements.     

Theoretical study of binding interactions and vibrational Raman spectra of water in hydrogen-bonded anionic complexes: (H2O)n- (n = 2 and 3), H2O...X- (X = F, Cl, Br, and I), and H2O...M- (M = Cu, Ag, and Au)

Binding interactions and Raman spectra of water in hydrogen-bonded anionic complexes have been studied by using the hybrid density functional theory method (B3LYP) and ab initio (MP2) method. In order to explore the influence of hydrogen bond interactions and the anionic effect on the Raman intensities of water, model complexes, such as the negatively charged water clusters ((H2O)n-, n = 2 and 3), the water...halide anions (H2O...X-, X = F, Cl, Br, and I), and the water-metal atom anionic complexes (H2O...M-, M = Cu, Ag, and Au), have been employed in the present calculations. These model complexes contained different types of hydrogen bonds, such as O-H...X-, O-H...M-, O-H...O, and O-H...e-. In particular, the last one is a dipole-bound electron involved in the anionic water clusters. Our results showed that there exists a large enhancement in the off-resonance Raman intensities of both the H-O-H bending mode and the hydrogen-bonded O-H stretching mode, and the enhancement factor is more significant for the former than for the latter. The reasons for these spectral properties can be attributed to the strong polarization effect of the proton acceptors (X-, M-, O, and e-) in these hydrogen-bonded complexes. We proposed that the strong Raman signal of the H-O-H bending mode may be used as a fingerprint to address the local microstructures of water molecules in the chemical and biological systems.     

Cooperativity between OH...O and CH...O hydrogen bonds involving dimethyl sulfoxide-H2O-H2O complex

The cooperativity between the O-H...O and C-H...O hydrogen bonds has been studied by quantum chemical calculations at the MP2/6-311++G(d,p) level in gaseous phase and at the B3LYP/6-311++G(d,p) level in solution. The interaction energies of the O-H...O and C-H...O H-bonds are increased by 53 and 58%, respectively, demonstrating that there is a large cooperativity. Analysis of hydrogen-bonding lengths, OH bond lengths, and OH stretching frequencies also supports such a conclusion. By NBO analysis, it is found that orbital interaction plays a great role in enhancing their cooperativity. The strength increase of the C-H...O H-bond is larger than that of the O-H...O H-bond due to the cooperativity. The solvent has a weakening effect on the cooperativity.