Centrosymmetric Hydrogen Bond Research Articles

NMR and DFT studies of monounsaturated and ω-3 polyunsaturated free fatty acids in the liquid state reveal a novel atomistic structural model of DHA

Structures of low-energy conformers of caproleic acid (CA; 10:1 cis-9), oleic acid (OA; 18:1 cis-9), α-linolenic acid (ALA; 18:3 cis-9,12,15 ω-3), eicosapentaenoic acid (EPA; 20:5 cis-5,8,11,14,17 ω-3), and docosahexaenoic acid (DHA; 22:6 cis-4,7,10,13,16,19 ω-3) in the liquid state, based on detailed 1D NOE and density functional theory calculations of 1H NMR chemical shifts are presented. Transient 1D NOE experiments with variable mixing time showed significant through-space proximity of the CH2–COOH protons and the terminal CH3– groups. Variable temperature 1H NMR experiments revealed strong intermolecular centro-symmetric cyclic hydrogen bond interactions of the carboxylic groups of the monounsaturated oleic and caproleic acids (δ(COOH) ∼ 12.0–12.4 ppm), ALA and EPA (δ(COOH) ∼ 11.0 ppm). On the contrary, the carboxylic proton of DHA is strongly shielded with δ(COOH) ∼ 8.5 ppm. DFT calculations were interpreted in terms of aggregates of dimerized fatty acids with parallel and antiparallel interdigitated structures. The antiparallel arrangement was found to be in excellent agreement with experimental 1D NOE NMR data. For the dimeric DHA, a flip-flop process between a classical intermolecular centro-symmetric hydrogen bond through carboxylic groups and a novel intramolecular hydrogen bond between the carboxylic group and the terminal ω-3 double bond is demonstrated.

Lack of the 'long-distance' dynamical co-operative interactions due to low symmetry of hydrogen-bonded malonic acid aggregates in molecular crystals.

The paper explains the relationship between the energy of hydrogen bonds and the distance between associated carboxyl groups of malonic acid (MA) molecules by means of infrared spectroscopic studies of crystals of its four isotopic varieties [CH2(COOH)2, h4-MA; CH2(COOD)2, d2c-MA; CD2(COOH)2, d2m-MA; CD2(COOD)2, d4-MA]. The effects associated with impact on the isotopic dilution and changes in the temperature of spectrum registration on the fine structures of the νO-H and νO-D bands were analyzed. MA molecular crystals are characterized by a tendency to spontaneous H/D isotopic exchange both within centrosymmetric hydrogen bond cycles and methylene groups. The mono- and polycrystalline spectra obtained in the infrared range of isotopically neat and isotopically diluted by deuterons do not indicate the occurrence of anomalous temperature evolution in the course of lowering their registration temperature to 77 K. Theoretical calculations did not give clear confirmation of the nature of the phenomena analyzed.

How far the vibrational exciton interactions are responsible for the generation of the infrared spectra of oxindole crystals?

Oxindole (indolin-2-one, Ox) is a unique and a crucial molecular system in spectroscopic studies. Indole is the core structure of many substances found in the human body (tryptophan, serotonin) and the indole alkaloids have highly differentiated pharmacological properties such as analgesic, anti-fever and anti-inflammatory. The Ox's structural results given in the Cambridge Structural Database revealed the existence of only one crystalline form of Ox, referred to the α-form. However, we have experimentally noticed the existence of two polymorphic forms during the crystallization of Ox. Furthermore, the significant spectral differences that we have observed in the solid state infrared spectra of these two forms additionally confirm the existence of the polymorphism phenomenon. Of the four polymorphic forms of Ox, two of them - α - and β-forms - were of particular interest. In the crystalline lattices of both polymorphs, we observed a similar pattern of molecular arrangements giving rise to the supramolecular synthon according to the terminology of Etter. Moreover, hydrogen bonds in the dimer of the α-form are found to be non-equivalent (non-centrosymmetric dimers), having a length of 2797Å and 2979Å, respectively. Comparatively, in the most densely packed crystalline structure of Ox, the β-form, the dimer is formed by a pair of almost identical intermolecular hydrogen bonds and consequently the crystals of β-form exhibited spectral properties typical to centrosymmetric hydrogen bond dimers. In addition, the spectroscopic studies that we have conducted to polymorphic forms of Ox, isotopically diluted with deuterium, show the dramatic influence of isotopic substitution in the hydrogen bridge on the infrared spectra of hydrogen bonding. Thus, the main goal of this work is the proposition of a theoretical approach that can describe the main features of the crystalline infrared spectra of the Ox polymorphs. The proposed approach is based on the phenomenon of the exciton coupling results directly from intermolecular interactions in the vibrationally excited state which leads to the delocalization of the excitation over the molecules in the lattice and to the Davydov splitting effect in the crystalline spectra.

Crystal structure of hepta-guanidinium nona-hydrogen bis-[α-hexa-molybdoplatinate(IV)] hepta-hydrate.

The title compound, (CH6N3)7H9[PtMo6O24]2·7H2O, containing the well-known Anderson-type heteropolyoxomolybdate, was obtained by recrystallization of its powdered guanidinium salt. The protonated O atoms in the polyanion were confirmed by electron-density maps, inter-polyanion hydrogen bonds and bond-valance sums (BVS). The {[H4.5PtMo6O24]2}(7-) polyanion is the same as that already characterized in K7[H4.5PtMo6O24]2·11H2O [space group P-1; Lee & Joo (2010 ▶). Acta Cryst. E66, i8-i9]. The heteropolyanions form inversion-generated dimers, {[H4.5PtMo6O24]2}(7-), held together by each of the four μ3-O-H⋯μ1-O, two μ2-O-H⋯μ2-O hydrogen bonds and one centrosymmetric μ3-O-H-μ3-O hydrogen bond. The H atom of the centrosymmetric hydrogen bond is located on an inversion centre. One guanidinium ion and one water mol-ecule are equally disordered about a twofold rotation axis.

Structural and Spectroscopic Study of 6,7-Dicyano-Substituted Lumazine with High Electron Affinity and Proton Acidity

The introduction of cyano groups into lumazine (pteridine-2,4-(1H,3H)dione) at the C6 and C7 positions enhances its electron affinity, proton acidity, and solubility in solvents. As a result, 6,7-dicyanolumazine (DCNLH2) forms charge transfer (CT) complexes with donors such as tetrathiafulvalene, 2,3,5,6-tetramethyl-1,4-phenylenediamine, and 3,3',5,5'-tetramethylbenzidine and readily dissociates a proton from the N1 nitrogen to form a monoanionic salt with tetrabutylammonium (TBA(+)). Crystal structures of the CT complexes consist of mixed stacks in which DCNLH2 interacts with donors in face-to-face configurations, but they form intermolecular hydrogen bonds differently depending on the donor type. In the TBA(+) salt, two deprotonated DCNLH(-) monoanions form a unique dianionic dimer connected by two centrosymmetric hydrogen bonds, N3-H···O-C2, which is electronically isolated by the presence of bulky TBA(+) countercations and the absence of a proton at the N1 hydrogen-bonding site. This dimer fluoresces yellowish green (fluorescence quantum yield Φ = 0.04). Because the DCNLH(-) anion only shows weak blue fluorescence in aqueous solution (Φ < 0.01), we suggest that the dimer formation is responsible for the fluorescence enhancement with a large emission band shift to the low-energy side.

Neutron scattering studies of K3H(SO4)2 and K3D(SO4)2: The particle-in-a-box model for the quantum phase transition

In the crystal of K(3)H(SO(4))(2) or K(3)D(SO(4))(2), dimers SO(4)···H···SO(4) or SO(4)···D···SO(4) are linked by strong centrosymmetric hydrogen or deuterium bonds whose O···O length is ≈2.50 Å. We address two open questions. (i) Are H or D sites split or not? (ii) Is there any structural counterpart to the phase transition observed for K(3)D(SO(4))(2) at T(c) ≈ 85.5 K, which does not exist for K(3)H(SO(4))(2)? Neutron diffraction by single-crystals at cryogenic or room temperature reveals no structural transition and no resolvable splitting of H or D sites. However, the width of the probability densities suggest unresolved splitting of the wavefunctions suggesting rigid entities H(L1/2)-H(R1/2) or D(L1/2)-D(R1/2) whose separation lengths are l(H) ≈ 0.16 Å or l(D) ≈ 0.25 Å. The vibrational eigenstates for the center of mass of H(L1/2)-H(R1/2) revealed by inelastic neutron scattering are amenable to a square-well and we suppose the same potential holds for D(L1/2)-D(R1/2). In order to explain dielectric and calorimetric measurements of mixed crystals K(3)D((1-ρ))H(ρ)(SO(4))(2) (0 ≤ ρ ≤ 1), we replace the classical notion of order-disorder by the quantum notion of discernible (e.g., D(L1/2)-D(R1/2)) or indiscernible (e.g., H(L1/2)-H(R1/2)) components depending on the separation length of the split wavefunction. The discernible-indiscernible isostructural transition at finite temperatures is induced by a thermal pure quantum state or at 0 K by ρ.

Catena-Poly[[(6-carboxypyrazine-2-carboxylato)lithium]-μ-aqua

The asymmetric unit of the title compound, [Li(C6H3N2O4)(H2O)]n, contains an LiI ion with a distorted trigonal–bipyramidal coordination environment. It is chelated by a singly protonated ligand mol­ecule via its heterocyclic N atom, by two O aoms, each donated by an adjacent carboxyl­ate group, and is further coordinated by a water O atom which acts as a bridge, forming a mol­ecular ribbon. A proton attached to one of the carboxyl­ate O atoms is situated on an inversion centre and forms a short centrosymmetric hydrogen bond, generating mol­ecular layers parallel to the ac plane. These layers are held together by weak O—H⋯O hydrogen bonds in which the coordinated water mol­ecules act as donors, whereas carboxyl­ate O atoms are acceptors.

Polarized IR Spectra of the Hydrogen Bond in Two Different Oxindole Polymorphs with Cyclic Dimers in Their Lattices

This article focuses on the problem of remarkably strong changes in the fine structure patterns of the ν(N-H) and ν(N-D) bands attributed to the hydrogen and deuterium bonds accompanying the phase transition, which occurs between two polymorphic forms of oxindole. The lattices of these two different crystals contain hydrogen-bonded cyclic dimers differ in their geometry parameters. The source of these differences in the polymorph spectral properties results from the geometry relations concerning the dimers constituting the lattice structural units. In the case of the "alpha" phase, the hydrogen bond lengths of the dimers differ by 0.18 Å. This leads to the "off-resonance exciton coupling" weakly involving the dimer hydrogen bonds. For the "beta" phase, with practically symmetric dimers in the lattice, the spectra become typical for centrosymmetric hydrogen bond systems due to the full resonance of the proton or deuteron vibrations.

Synthesis and Crystal Structure of Tetrabutylammonium 4-(2-Bromothiophene-5-Sulfanilamide)-Benzoate

Tetrabutylammonium 4-(2-bromothiophene-5-sulfanilamide)-benzoate crystallizes in C2/C lattice with a = 18.3389(17), b = 16.540(16), c = 15.8280(15) A, β = 116.959(2), V = 4279.5(7) A3, Z = 4, and R = 0.0560. In the structure a very short centrosymmetric hydrogen bond and C=O···Br halogen bond interactions, together with N–H···O and C–H···O hydrogen bonds give a three-dimensional network. A extremely short O–H···O hydrogen bond is observed between the two neighboring 4-(2-bromothiophene-5-sulfanilamide)-benzoates. The H atom is fixed at the crystallographic inversion center and bridges the two benzoates to form an anion dimer. Halogen bonds also aid in forming the interest structure in crystal.

Effects of strong inter-hydrogen bond dynamical couplings in the polarized IR spectra of adipic acid crystals

This paper presents the results of the re-investigation of polarized IR spectra of adipic acid and of its d 2, d 8 and d 10 deuterium derivative crystals. The spectra were measured at 77 K by a transmission method using polarized light for two different crystalline faces. Theoretical analysis concerned linear dichroic effects and H/D isotopic effects observed in the spectra of the hydrogen and deuterium bonds in adipic acid crystals at the frequency ranges of the ν O–H and the ν O–D bands. The two-branch fine structure pattern of the ν O–H and ν O–D bands and the basic linear dichroic effects characterizing them were ascribed to the vibronic mechanism of vibrational dipole selection rule breaking for IR transitions in centrosymmetric hydrogen bond dimers. It was proved that for isotopically diluted crystalline samples of adipic acid, a non-random distribution of protons and deuterons occurs in the dimers (H/D isotopic “ self-organization” effect). This effect results from the dynamical co-operative interactions involving the dimeric hydrogen bonds.

New polymorphic modification of the crystal structure of bromopeganol

The crystal structure of 6-bromopeganol in a new polymorphic modification is found and determined by single crystal X-ray diffraction analysis. For the crystal structure of the new polymorph the formation of a centrosymmetric closed dimer (associate) is also characteristic. It consists of two bromopeganol molecules bound by centrosymmetric reciprocal hydrogen bonds O-H...N(1). This polymorph differs from the known one in the mutual arrangement and interaction of these associates.

An anomalous linear dichroic effect in the polarized IR spectra of 2-furancarboxylic acid crystals: Proton transfer induced by co-operative interactions involving hydrogen bonds

The polarized IR spectra of the hydrogen bond systems were studied in crystals of 2-furancarboxylic acid C 4H 3O–COOH as well as in crystals of its deuterium-bonded derivative. The spectra were measured by a transmission method at room temperature and at 77 K. Theoretical analysis of the results concerned the linear dichroic effects, the H/D isotopic and the temperature effects observed in the solid-state IR spectra at the frequency ranges of the ν O–H and the ν O–D bands, respectively. The main spectral properties of the crystals can be quantitatively interpreted in terms of the “ strong-coupling” theory for a hydrogen bond dimer model. This model explains satisfactorily not only the two-branch structure of the ν O–H and the ν O–D bands, the temperature-induced evolution of the crystalline spectra, but also the basic linear dichroic effects observed in the band frequency ranges. A vibronic mechanism, responsible for promotion of the symmetry-forbidden transition in the IR and for the excitation of the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers was also taken into the account. An excess of fine linear dichroic effects, accompanying the splitting of the spectral lines in the crystalline spectra, appeared for the crystal with only one dimer in a unit cell. This fact suggests that the existence of a specific co-operative interaction effect in the lattice, depending on the stimulated rearrangement of the proton positions in the hydrogen bond dimers, occurring in the nearest surrounding of an individual dimer in the lattice. Therefore, regardless of the published X-ray P 1 ¯ space-symmetry of the lattice, the effective unit cell for explaining the crystalline spectra behaved as if it contained two nonequivalent dimers. Re-examination of the crystal X-ray structure supported the hypothesis concerning the lattice transformation predicted by the IR spectroscopic studies.

Inter-hydrogen bond dynamical coupling effects in polarized IR spectra of N-methylacetamide single crystals

This article presents the investigation results of polarized IR spectra of the hydrogen bond in N-methylacetamide (NMA) crystals measured in the frequency range of the proton and deuteron stretching vibration bands, ν N–H and ν N–D. A similar study was also performed for crystals of the deuterium isotopomers of the compound, D 7-NMA (CD 3CONDCD 3) and D 6-NMA (CD 3CONHCD 3). On the basis of the analysis of the linear dichroic and temperature effects, the two-branch structure of the ν N–H bands in the spectra was ascribed to centrosymmetric hydrogen bond pairs in the lattice. Each hydrogen bond in such a dimeric system belonged to another chain of the associated molecules. The exciton interactions involving the dimer hydrogen bonds were considered to be responsible for the band shape generation. For the deuterium-bonded crystals the exciton interactions were found to be weaker since the ν N–D bands were less split. Within an individual hydrogen or deuterium bond chain the in-chain exciton couplings involving hydrogen bonds were estimated as considerably weaker than the inter-chain ones. The exciton dilution retains the two-branch fine structure pattern of the “ residual” ν N–H and ν N–D bands. This means that the inter-chain couplings involving hydrogen bonds do not change, when the in-chain couplings vanish. These results are the evidence of the influence of non-conventional co-operative interactions occurring in the hydrogen bond systems on the spectra. These co-operative interactions are responsible for the non-random distribution of the hydrogen isotope atoms in the hydrogen bridge lattices, namely for the grouping of identical hydrogen isotope atoms in the dimers. The proposed interpretation of the IR spectra of the hydrogen bond in N-methylacetamide (NMA) crystals casts light on the spectra generation mechanisms and gives a new meaning to the traditional nomenclature applied for describing the ν N–H band structure pattern in IR spectra of amides.

“ Reversal” exciton coupling effect in the IR spectra of the hydrogen bond cyclic dimers; polarized IR spectra of 3-hydroxy-4-methyl-2(3H)-thiazolethione crystals

This paper presents the results of our investigation of the polarized IR spectra of the hydrogen bond in crystals of 3-hydroxy-4-methyl-2(3H)-thiazolethione. These spectra were measured at room temperature and at 77 K by a transmission method using polarized light. The spectral studies were preceded by re-determination of the crystal X-ray structure. Theoretical analysis of the results concerned linear dichroic effects, the H/D isotopic and temperature effects observed in the solid-state IR spectra of hydrogen and deuterium bonds at frequency ranges of the ν O H and the ν O D bands, respectively. Basic spectral properties of the crystals can be interpreted satisfactorily in terms of the “ strong-coupling” theory, when based on a hydrogen bond dimer model. This model sufficiently explains not only the two-branch structure of the ν O H and the ν O D bands and the temperature-induced evolution of the crystalline spectra, but also the linear dichroic effects observed in the band frequency ranges. A vibronic mechanism responsible for promotion of the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers was analyzed. The crystal spectral properties were successfully interpreted on assuming a “ reversal” Davydow-splitting effect expressed by changing of the vibrational exciton interaction energy sign for hydrogen bond dimers in the lattice. These effects were ascribed to a considerable elongation of the dimeric O H ⋯ S bonds in the crystal due to the large atomic radius of sulfur determining the dimer geometry.

Study of polarized IR spectra of the hydrogen bond system in crystals of styrylacetic acid

We have investigated the polarized IR spectra of the hydrogen bond system in crystals of trans-styrylacetic acid C 6H 5 CH CH CH 2 COOH, and also in crystals of the following three deuterium isotopomers of the compound: C 6H 5 CH CH CH 2 COOD, C 6H 5 CH CH CD 2 COOH and C 6H 5 CH CH CD 2 COOD. The spectra were measured at room temperature and at 77 K by a transmission method. The spectral studies were preceded by determination of the X-ray crystal structure. Theoretical analysis of the results concerned linear dichroic effects, the H/D isotopic and temperature effects, observed in the solid-state IR spectra of the hydrogen and of the deuterium bond, at the frequency ranges of the ν O H and the ν O D bands, respectively. Basic spectral properties of the crystals can be interpreted satisfactorily in terms of the “ strong-coupling” theory, when based on a hydrogen bond dimer model. This model sufficiently explained not only a two-branch structure of the ν O H and the ν O D bands, and temperature-induced evolution of the crystalline spectra, but also the linear dichroic effects observed in the band frequency ranges. A vibronic mechanism was analyzed, responsible for promotion of the symmetry-forbidden transition in the IR for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers. It was found to be of minor importance, when compared with analogous spectral properties of arylcarboxylic acid, or of cinnamic acid crystals. These effects were ascribed to a substantial weakening of electronic couplings between the hydrogen bonds of the associated carboxyl groups and the styryl radicals, associated with the separation of these groups in styrylacetic acid molecules by methylene groups in the molecules.

Self-association and stereochemistry study of (5R)-4-methyl-5-phenyl-1,3,4-oxadiazinan-2-one

Abstract C10H12N2O2, M r = 191.21, P21, a = 5.5771(7), b = 27.841(3), c = 6.906(1) Å, β = 111.937(9)°, Z = 4, R 1 = 0.0462. The two independent molecules in the asymmetric unit are linked through two very strong quasi centro-symmetric hydrogen bonds. The oxadiazinone rings, of both molecules, show the pyramidal inversion at N-4 nitrogen, thus resulting in a C(R)N(S)—C(R)N(R) diastereomeric pair. The analysis of the IR N-H stretching band (in the solid and in carbon tetrachloride solution) and the 13C NMR experiment at low temperature are in agreement with the X-ray crystal structure.

Classical hydrogen bonding and weaker C–H⋯O interactions in complexes of uracil-5-carboxylic acid with the alkali metals Na–Cs

Reaction of uracil-5-carboxylic acid with M 2CO 3 or M(OH) (M = Na–Cs) gave five new polymeric coordination compounds: [Na(C 5H 3N 2O 4)(H 2O)] ∞ ( 1); [K(C 5H 3N 2O 4)(C 5H 4N 2O 4)] ∞ ( 2); [Rb{H(C 5H 3N 2O 4) 2} · 2H 2O] ∞ ( 3); [Rb(C 5H 3N 2O 4)(H 2O)] ∞ ( 4); [Cs{H(C 5H 3N 2O 4) 2}(H 2O) 2] ∞ ( 5). Characterisation by single-crystal X-ray diffraction using both standard laboratory equipment and high-flux synchrotron radiation revealed that very different hydrogen bonding networks are formed in each case. Hydrogen-bonded chains ( 1), sheets ( 2 and 5) and tapes ( 3 and 4) are all found in these compounds. All except 3 additionally contain C–H⋯O interactions as an integral part of the hydrogen bonding motifs formed. Compounds 3 and 5 also feature centrosymmetric hydrogen bonds as a result of partial deprotonation, whereby a single hydrogen atom is shared equally between two carboxylate groups.

Study of hydrogen bond polarized IR spectra of cinnamic acid crystals

This paper presents the results of investigation of the polarized IR spectra of cinnamic acid and of its deuterium derivative crystals. The spectra were measured by a transmission method, using polarized light, at the room temperature and at 77 K, for two different crystalline faces. Theoretical analysis of the results concerned linear dichroic effects, H/D isotopic and temperature effects, observed in the spectra of the hydrogen and of the deuterium bonds in cinnamic acid crystals, at the frequency ranges of the ν O–H and the ν O–D bands. The basic crystal spectral properties could be satisfactorily interpreted in a quantitative way for a centrosymmetric cyclic hydrogen bond dimer model. Such a model explains not only a two-branch structure of the ν O–H and ν O–D bands in crystalline spectra, but also some essential linear dichroic effects in the band frequency ranges, measured for isotopically diluted crystals. Model calculations, performed within the limits of the ‘strong-coupling’ model, allowed for quantitative interpretation and for understanding of the basic properties of the hydrogen bond IR spectra of cinnamic acid crystals, H/D isotopic, temperature and dichroic effects included. In the scope of our studies the mechanism of H/D isotopic ‘self-organization’ processes, taking place in the crystal hydrogen bond lattices, was also recognized. It was proved that for isotopically diluted crystalline samples of cinnamic acid, a non-random distribution of protons and deuterons occurs exclusively in the hydrogen bond dimers. Nevertheless, these co-operative interactions between the hydrogen bonds do not involve the adjacent hydrogen bond dimers in each unit cell. The two-branch fine structure pattern of the ν O–H and ν O–D bands was ascribed to the vibronic mechanism of vibrational dipole selection rule breaking in centrosymmetric hydrogen bond dimers. The observed in the spectra very high intensity of the forbidden transition sub-band in the analyzed ν O–H and ν O–D bands is a manifestation of an extremely effective symmetry rule breaking mechanism. It correlates with a relatively large excess electron charge on the cinnamic aid dimer carboxyl groups. This effect is a result of a partial withdrawal of the electron charge, from the conjugated π-bond systems of the styryl substituents, by the carboxyl groups. This statement has been supported by ab initio calculations.

Polarization IR spectra of the hydrogen bond in 1-naphthylacetic and 2-naphthylacetic acid crystals: H/D isotopic effects. Temperature and polarization effects

This paper deals with experimental studies and with the quantitative interpretation of the polarized IR crystalline spectra of 1-naphthylacetic and 2-naphthylacetic acid. The spectra were measured in the ν O–H and in the ν O–D band frequency ranges, at the temperatures of 298 and 77 K. The intensity distribution in the bands was quantitatively reproduced on the basis of the strong-coupling model, when assuming that the isolated (COOH) 2 and (COOD) 2 cycles were the source of the spectral properties of the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. A vibronic mechanism, promoting the symmetry forbidden transition in the IR, for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers, was found to be of a considerably minor importance, when compared with analogous spectral properties of arylcarboxylic acid crystals. Also no Fermi resonance impact on the ν O–H band shape was identified in the 1-naphthylacetic and 2-naphthylacetic acid crystal spectra. These effects were ascribed to weakening of electronic couplings between the hydrogen bonds and the phenyl rings, due to the separation of these molecular fragments by methylene groups in 1-naphthylacetic and 2-naphthylacetic acid molecules.

Bis( N-methylpiperidine betaine) hydrobromide: crystal structure and hydrogen bonding

Bis( N-methylpiperidine betaine) hydrobromide, (MPB) 2HBr, was synthesised and its crystal structure has been solved by X-ray diffraction at 100 K and refined to R=0.020. The crystals are monoclinic, space group C2/ c, a=22.885(2), b=7.4498(5), c=10.6896(7) Å, β=92.89(1)°, V=1820.1(2) Å 3, Z=4. The piperidine ring adopts a chair conformation with the N +–CH 2COO·H group in an equatorial and the N +–CH 3 group in an axial position. A pair of betaine molecules are bridged by a proton to form a dimeric cation featuring a very short centrosymmetric hydrogen bond (O⋯O distance 2.445(2) Å). FTIR spectra of (MPB) 2HBr and (MPB) 2HCl show very similar broad absorption in the 1000–500 cm −1 region typical for very short hydrogen bonds. In the second and fourth derivative spectra, the carbonyl band appears as a single band. PM3 calculations for the dimeric cation reproduced a hydrogen bond that is much shorter than in the crystal and slightly asymmetric, while B3LYP predicts O⋯O distance that is comparable to that in the crystal, and corresponding to a symmetrical or asymmetrical hydrogen bond, depending on the conformation of the carboxymethyl substituent (equatorial–equatorial or axial–axial, respectively).