Cyclic Hydrogen-bonded Structure Research Articles

Strong Intermolecular Vibrational Coupling through Cyclic Hydrogen-Bonded Structures Revealed by Ultrafast Continuum Mid-IR Spectroscopy

Cyclic hydrogen-bonded structures are common motifs in biological systems, providing structural stability and mediating proton transfer for redox reactions. The mechanism of proton transfer across hydrogen-bonded interfaces depends on the strength of the intermolecular coupling between bridging OH/NH vibrational modes. Here we present a novel ultrafast continuum mid-IR spectroscopy experiment to study the vibrational dynamics of the 7-azaindole-acetic acid (7AI-Ac) heterodimer as a model system for asymmetric cyclic hydrogen-bonded structures. In addition to spreading of the excitation across the whole OH band within the time resolution of the experiment, excitation of a 300 cm(-1) region of the ∼1000 cm(-1) broad OH stretching mode of the acetic acid monomer leads to a frequency shift in the NH stretching mode of the 7AI monomer. This indicates that the NH and OH stretching modes located on the two monomers are strongly coupled despite being separated by 750 cm(-1). The strong coupling further causes the OH and NH bands to decay with a common decay time of ∼2.5 ps. This intermolecular coupling is mediated through the hydrogen-bonded structure of the 7AI-Ac heterodimer and is likely a general property of cyclic hydrogen-bonded structures. Characterizing the vibrational dynamics of and the coupling between the high-frequency OH/NH modes will be important for understanding proton transfer across such molecular interfaces.

Entropy-driven rearrangement of the water network at the hydrated amide group of the trans-formanilide–water cluster in the gas phase

Photoionization-induced rearrangement of the water network in the trans-formanilide 1:4 cluster, FA-(H(2)O)(4), has been investigated by using IR-photodissociation spectroscopy and quantum chemical calculations. The IR spectrum of FA-(H(2)O)(4) in the S(0) state shows that the observed cluster has a cyclic hydrogen-bonded structure where the CO group and the NH group of FA are bridged with four water molecules, consistent with the reported structure [E. G. Robertson, Chem. Phys. Lett., 2000, 325, 299]. However, the corresponding cyclic hydrogen-bonded structure in the D(0) state of [FA-(H(2)O)(4)](+) is a minor product arising from photoionization via the S(1)-S(0) origin of FA-(H(2)O)(4). The dominant product has an extended H-bonded structure, where the intermolecular hydrogen bond between the hydrogen of the OH group of a water molecule and the CO group is dissociated. This is the first observation of a photoionization-induced rearrangement of the water network in [FA-(H(2)O)(4)](+). Through DFT calculations, we conclude that the rearrangement occurs due to entropic effects.

Excited-State Triple-Proton Transfer in 7-Azaindole(H2O)2 and Reaction Path Studied by Electronic Spectroscopy in the Gas Phase and Quantum Chemical Calculations

We have investigated the excited-state multiple-proton/hydrogen atom transfer reactions in the 7-azaindole water clusters, [7AI](H(2)O)(n) (n = 2,3), in the gas phase by combining electronic spectroscopy and quantum chemical calculations. The fluorescence excitation (FE) spectrum of 7AI(H(2)O)(2) has been observed by monitoring visible emission. In contrast, no vibronic bands are detected in the FE spectrum of 7AI(H(2)O)(3) when the visible emission is monitored. The dispersed fluorescence spectra of 7AI(H(2)O)(n) (n = 2,3) have been measured. The excitation of +180 cm(-1) band from the electronic origin of 7AI(H(2)O)(2) enhances the visible emission as compared with the 0-0 excitation. The +180 cm(-1) band is assgined to an intermolecular mode (σ(1)) of the cyclic hydrogen-bonded ring structure. The calculated S(1)-S(0) absorption spectrum for the cyclic hydrogen-bonded structure is in agreement with the FE spectrum around the 0-0 region. The excitation of σ(1) significantly promotes the reaction and generates the tautomeric form of 7AI(H(2)O)(2). These experimental results on 7AI(H(2)O)(n) (n = 2,3) are very similar to those on 7AI(CH(3)OH)(n) (n = 2,3) and 7AI(C(2)H(5)OH)(n) (n = 2,3). We conclude that the excited-state triple proton/hydrogen atom transfer (ESTPT/HT) occurs in 7AI(H(2)O)(2). Cuts of the potential energy surfaces along the proton/hydrogen atom transfer coordinates of 7AI(H(2)O)(n) (n = 2,3) and 7AI(CH(3)OH)(n) (n = 2,3) are comparatively calculated by quantum chemistry calculations (RI-CC2/cc-pVDZ and TD-DFT(B3LYP)/cc-pVDZ) to explore the mechanism of the ESTPT/HT reaction. The calculated results suggest that concerted proton transfers occur in 7AI(H(2)O)(2) as well as in 7AI(CH(3)OH)(2), whereas the potential barrier for the excited-state quadruple proton transfer in 7AI(H(2)O)(3) and 7AI(CH(3)OH)(3) is higher than those for ESTPT. The theoretical results are consistent with the observation of ESTPT/HT in 7AI(H(2)O)(2).

Hydrogen-bonded structures of pyrrole-solvent clusters: Infrared cavity ringdown spectroscopy and quantum chemical calculations

The hydrogen-bonded structures of pyrrole-solvent (H(2)O,CH(3)OH,C(2)H(5)OH) binary clusters were studied by the combination of experimental and theoretical techniques. Infrared cavity ringdown spectroscopy was applied to observe the NH and OH stretching vibrations of the clusters. The structures, binding energies, and normal modes of the binary clusters were obtained by quantum chemical calculations of the MP2/6-31+G(d,p) and B3LYP/6-311+G(d,p) levels. For the 1:1 clusters of pyrrole-H(2)O, pyrrole-CH(3)OH, and pyrrole-C(2)H(5)OH, the hydrogen-bonded NH stretching vibrations were observed at 3448, 3414, and 3408 cm(-1), respectively. They were redshifted from the NH stretching vibration of the pyrrole monomer, and the amounts of the redshift were proportional to the proton affinities of the solvent molecules. MP2 level calculations revealed that the sigma-type (NH...O) hydrogen-bonded structures had 7.6-9.0 kJ/mol larger binding energies than the pi-type structures (OH...pi electron cloud of pyrrole), and that the vibrational frequencies of the sigma-type structures are consistent with the observed spectra. In addition to the 1:1 clusters, the NH or OH stretching vibrations of pyrrole-CH(3)OH binary clusters were observed at 3432 and 3549 cm(-1). Among three optimized structures of the pyrrole-(CH(3)OH)(2), the sigma-pi bridge pyrrole-(CH(3)OH)(2) provided a reasonable agreement between the observed and calculated vibrational frequencies. For the pyrrole-H(2)O binary clusters, three new bands were observed at 3414, 3435, and 3541 cm(-1). These bands are consistent with the calculated NH and OH stretching vibrations of the (pyrrole)(2)-H(2)O cluster, which has a closed cyclic hydrogen-bonded structure.

Selective cleavage of the C–O bonds in alcohols and asymmetric ethers by dissociative electron attachment

Dissociative electron attachment spectra of 20 saturated compounds containing ether and hydroxyl groups are presented. Two groups of fragmentation processes are identified: (i) one or two bands mediated by shape resonances in the 1-5 eV range and (ii) three bands (often overlapping) in the 5-12 eV range, assigned to Feshbach resonances with a hole in either the oxygen 'nonbonding' orbitals n(O) or [symbol: see text], or in one of the available sigma orbitals. The main result of this paper is the discovery of unexpected selectivity in the cleavage of C-O bonds in asymmetric ethers, R1-O-R2, within the range of the sigma-Feshbach resonances, where the loss of a neutral alkyl group, R1, peaks at a given energy (9.1, 8.4, 8.0 and 8.8 eV for R1 = ethyl, propyl, butyl and t-butyl, respectively), independently of R2 in all compounds studied. This empirical rule indicates an unexpected conclusion, that the excitation (a hole and two excited electrons) of Feshbach resonances responsible for the fragmentation is spatially localized on one alkyl group or the other. This interpretation is supported by correlation with ionization energies of the alkanes, R1H. The methyl group behaves differently from the larger alkyl groups-it is not split off at all (except in methanol), thus acting as a 'protective group'. A number of other observations were made: the previously observed lack of cleavage of C-O bonds (in contrast to O-H bonds) in the n(O) and [symbol: see text] Feshbach resonance bands was confirmed in all compounds containing one oxygen atom, but it is now shown that the C-O bond can be broken under certain circumstances in compounds with two oxygen atoms, either in cyclic hydrogen-bonded structures or when the neutral fragment contains oxygen.

Ab initio and Vibrational Predissociation Studies on Methylammonium-(Water)4 Complex: Evidence for Multiple Cyclic and Non-cyclic Hydrogen-bonded Structures

The combined ab initio and vibrational predissociation (VP) spectroscopic studies on methylammonium(water)4 complex aimed at understanding the hydration behavior of an amphiphilic ion core are described. The ab initio calculations predicted eleven low-energy isomers forming cyclic, tripod, chain, and caged structures, and their relative stabilities, total hydration energies and thermodynamic functions at 298 K and 150 K. The excellent correlation between the observed VP spectra and ab initio spectra for bonded N-H, bonded O-H and free O-H stretches suggested co-existence of five cyclic isomers and two non-cyclic isomers in ion beam at 150 K, consistent with the trends of calculated Gibbs free energies.

Urea and Urea Nitrate Decomposition Pathways: A Quantum Chemistry Study

Electronic structure calculations have been performed to investigate the initial steps in the gas-phase decomposition of urea and urea nitrate. The most favorable decomposition pathway for an isolated urea molecule leads to HNCO and NH3. Gaseous urea nitrate formed by the association of urea and HNO3 has two isomeric forms, both of which are acid-base complexes stabilized by the hydrogen-bonding interactions involving the acidic proton of HNO3 and either the O or N atoms of urea, with binding energies (D0(o), calculated at the G2M level with BSSE correction) of 13.7 and 8.3 kcal/mol, respectively, and with estimated standard enthalpies of formation (delta(f)H298(o) of -102.3 and -97.1 kcal/mol, respectively. Both isomers can undergo relatively facile double proton transfer within cyclic hydrogen-bonded structures. In both cases, HNO3 plays a catalytic role for the (1,3) H-shifts in urea by acting as a donor of the first and an acceptor of the second protons transferred in a relay fashion. The double proton transfer in the carbonyl/hydrogen bond complex mediates the keto-enol tautomerization of urea, and in the other complex the result is the breakdown of the urea part to the HNCO and NH3 fragments. The enolic form of urea is not expected to accumulate in significant quantities due to its very fast conversion back to H2NC(O)NH2 which is barrierless in the presence of HNO3. The HNO3-catalyzed breakdown of urea to HNCO and NH3 is predicted to be the most favorable decomposition pathway for gaseous urea nitrate. Thus, HNCO + NH3 + HNO3 and their association products (e.g., ammonium nitrate and isocyanate) are expected to be the major initial products of the urea nitrate decomposition. This prediction is consistent with the experimental T-jump/FTIR data [Hiyoshi et al. 12th Int. Detonation Symp., Aug 11-16, San Diego, CA, 2002].

Chemistry of Styrene (Water)nClusters,n= 1−5: Spectroscopy and Structure of the Neutral Clusters, Deprotonation of Styrene Dimer Cation, and Implication to the Inhibition of Cationic Polymerization

The styrene-water binary clusters SW(n), with n = 1-5 have been studied by the (one-color) resonant two-photon ionization technique using the resonance of styrene. The structures and energetics of the neutral clusters are investigated using a search technique that employs Monte Carlo procedure. The strong tendency for water molecules to form cyclic hydrogen-bonded structures is clearly observed in the SW(n) structures starting from n =3. The results indicate that the spectral shifts correlate with the interaction energies between styrene and the water subcluster (W(n)) within the SW(n) clusters. Evidence is presented that points to (1) the formation of a covalent bonded styrene radical cation dimer following the 193 nm MPI of styrene neutral clusters, (2) proton transfer from the styrene dimer cation to the water or methanol subcluster, resulting in the formation of protonated water or methanol clusters and a styrene dimer radical, and (3) extensive solvation of the styrene dimer radical within the protonated solvent molecules. The proton-transfer reactions may explain the strong inhibition effects exerted by small concentrations of water or methanol on the cationic polymerization of styrene. These results provide a molecular level view of the inhibition mechanism exerted by protic solvents on the cationic polymerization of styrene.

FTIR studies of the interactions of 1,3,5-triazabicyclo[4.4.0]dec-5-ene with 4-tert-butylphenol and 4-cyanophenol

The formation of hydrogen-bonded complexes in the mixtures of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) with 4-tert-butylphenol and 4-cyanophenol has been studied as a function of the phenol concentration in chloroform and acetonitrile by FTIR spectroscopy. In chloroform, the formation of cyclic hydrogen-bonded structures between the TBD and phenol molecules was observed. These structures show large proton polarizability due to collective proton motion within these hydrogen bonds indicated by continuous absorptions in the FTIR spectra. In acetonitrile, the cyclic hydrogen-bonded structure is relatively stable in the 1:1 mixtures. With increasing concentration of phenol molecules the formation of non-cyclic hydrogen-bonded chains is much more favorable.

Modeling H-bonding and solvent effects in the alkylation of pyrimidine bases by a prototype quinone methide: a DFT study.

Nucleophilicity of NH(2), N3, and O(2) centers of cytosine toward a model quinone methide (o-QM) as alkylating agent has been studied using DFT computational analysis [at the B3LYP/6-311+G(d,p) level]. Specific and bulk effects of water (by C-PCM model) on the alkylation pathways have been evaluated by analyzing both unassisted and water-assisted reaction mechanisms. An ancillary water molecule, H-bonded to the alkylating agent, may interact monofunctionally with the o-QM oxygen atom (passive mechanisms) or may participate bifunctionally in cyclic hydrogen-bonded structures as a proton shuttle (active mechanisms). A comparison of the unassisted with the water-assisted reaction mechanisms has been made on the basis of activation Gibbs free energies (DeltaG(++)). The gas-phase alkylation reaction at N3 does proceed through a passive mechanism that is preferred over both the active (by -6.3 kcal mol(-1)) and the unassisted process. In contrast, in the gas phase, the active assisted processes at NH(2) and O(2) centers are both favored over their unassisted counterparts by -4.0 and -2.2 kcal mol(-1), respectively. The catalytic effect of a water molecule, in gas phase, reduces the gap between the TSs of the O(2) and NH(2) reaction pathways, but the former remains more stable. Water bulk effect significantly modifies the relative importance of the unassisted and water-assisted alkylation mechanisms, favoring the former, in comparison to the gas-phase reactions. In particular, the unassisted alkylation becomes the preferred mechanism for the reaction at both the exocyclic (NH(2)) and the heterocyclic (N3) nitrogen atoms. By contrast, alkylation at the cytosine oxygen atom is a water-catalyzed process, since in water the active water-assisted mechanism is still favored. As far as competition, among all the possible mechanisms, our calculations unambiguously suggest that the most nucleophilic site both in gas phase (naked reagents: N3 >> O(2) >or= NH(2)) and in water solution (solvated reagents: N3 >> NH(2) >> O(2)) is the heterocyclic nitrogen atom (N3) (DeltaG(++)(gas) = +7.1 kcal mol(-1), and DeltaG(++)(solv) = +13.7 kcal mol(-1)). Our investigation explains the high reactivity and selectivity of the cytosine moiety toward o-QM-like structures both in deoxymononucleoside and in a single-stranded DNA, on the basis of strong H-bonding interactions between reactants and solvent bulk effect. It also offers two general reactivity models in water, uncatalyzed and active water-catalyzed mechanisms (for nitrogen and oxygen nucleophiles, respectively), which should provide a general tool for the planning of nucleic acid modification.

Structural effects on the formation of proton and alkali metal ion adducts of apolar, neutral peptides: electrospray ionization mass spectrometry and Ab initio theoretical studies.

Apolar, neutral peptides have been shown to ionize extremely well under the conditions used for electrospray ionization mass spectrometry (ESIMS). Peptides for which the conformations have been independently determined in solution and in crystals have been examined by ESIMS. Studies of peptide helices ranging from 7 to 18 residues reveal that shorter helices yield exclusively singly charged ions, while in larger helices multiply charged species are detectable. Multiple sites for protonation/metallation are introduced in the helix by proline insertion or by changing the chirality in the residue. The preferred site of cation binding to helices may be the C-terminus end, where three free CO groups are available for chelation. Ab initio and DFT calculations at several levels have been carried out for the binding of H+, Li+, Na+, and K+ to CHO-(Gly)3)-OMe. The results reveal that metallation in helices is favoured by chelation to carbonyl groups at the C-terminus, while protonation involved two carbonyl groups and thus favour a 10-membered cyclic hydrogen-bonded structure. In -strands, metallation/protonation occurs at isolated carbonyl groups. Collision induced fragmentation of hydrophobic peptides under ESI conditions reveals that helix fragmentation occurs predominantly from the C-terminus, while in -hairpins cleavage occurs simultaneously at multiple sites.

Molecular and Supramolecular Structures ofN-Phenyl Formamide and its Hydrated Clusters

The amide, N-phenyl formamide (formanilide), and its water clusters have been studied in a jet expansion using laser-induced fluorescence excitation and mass-selected, resonant two-photon ionization (R2PI) techniques. The isomer with a trans configuration of the amide group (defined in Figure 1) is identified through analysis of the partially resolved contour of its S1 ← S0 band origin. Ion-dip “hole-burn” spectra of the nonplanar cis isomer contain either symmetric or antisymmetric components of low-frequency progressions, providing evidence of a double-minimum ground state potential. Excited-state vibrations at 76 and 152 cm-1, which are strongly Franck−Condon active, show evidence of Duschinski mixing of the ground-state modes including Cring−N torsion. Water clusters have been observed for trans-formanilide only: two distinct 1:1 hydrates, two 1:2 hydrates, and a complex with at least four bound water molecules. (The cis isomer is also expected to form extremely stable complexes with water, but none have been detected experimentally in the present study.) The observed clusters are assigned using spectroscopic data, including band contours, to structural alternatives computed ab initio at the HF/6-31G* level. The 1:1 hydrates are assigned to a cluster in which water binds at the NH site and one in which water binds at the HCO site. In the 1:2 clusters, the addition of a further water molecule to each of the 1:1 clusters results in cyclic hydrogen-bonded structures, with the water dimer bridging between proton donor and proton acceptor sites of the host. The interactions are HCO···HOH and OCH···OH2 in one case and NH···OH2 and πring···HOH in the other. At the MP2/6-31G*//HF/6-31G* level, these structures are ca. 10 kJ mol-1 more stable than the nearest competitor, in part because of cooperative effects. The R2PI spectrum of the NH bound 1:2 cluster “C” is very similar to that of the indole(H2O)2 complex assigned by Zwier and co-workers.19 Its origin is red-shifted 482 cm-1 from trans-formanilide, and the electronic transition excites long intermingled vibrational progressions with frequencies of 29, 38, and 51 cm-1.

Hydrated Clusters of 2-Phenylethyl Alcohol and 2-Phenylethylamine: Structure, Bonding, and Rotation of the S1 ← S0 Electronic Transition Moment

Hydrated clusters of 2-phenylethyl alcohol (PEAL) and 2-phenylethylamine (PEA) have been studied in a jet-cooled environment, using laser-induced fluorescence excitation and mass-selected resonant two-photon ionization (R2PI) spectroscopy of the S1 ← S0 electronic transitions. Spectral features have been observed for clusters M(H2O)n, n = 1−4, and their stoichiometry assigned on the basis of the ion fragmentation patterns. Ionization of hydrated PEA(H2O)n clusters leads to the observation of PEA(H2O)n-1+ and CH2NH2(H2O)n+ ions. Partially resolved rotational band contours of several n = 1, 2 clusters have been analyzed with the aid of ab initio molecular orbital calculations, conducted at the MP2/6-31G*//HF/6-31G* level for the ground state, and CIS/6-31G* for the first electronically excited singlet state. The analysis reveals the supramolecular structure: the host molecular conformation within these clusters and the binding sites of the water molecules. In n = 1 clusters of 2-phenylethylamine, the primary binding site involves hydrogen bonding to the nitrogen atom in the amine group. Cyclic hydrogen-bonded structures are observed for n = 2 clusters. In 2-phenylethyl alcohol, two different 1:1 clusters have been assigned in which the water molecule binds alternatively as a proton acceptor and proton donor. Further interactions between water molecules and the host, e.g., Hwater···π and Owater···HC, lead to additional stabilization of certain complexes. The assignments are aided greatly by the extraordinary sensitivity of the S1 ← S0 transition moment alignment to both side chain conformation and long-range intermolecular interactions.

Molecular structure and dynamics of C-1-adamantyl substituted N-unsubstituted pyrazoles studied by solid state NMR spectroscopy and X-ray crystallography

The influence of the 1-adamantyl group on the structure and the proton transfer dynamics of N-unsubstituted pyrazoles has been determined. Four compounds have been labelled with 15 N and studied by variable temperature 15 N CP MAS NMR spectroscopy: 3(5)-(1-adamantyl)pyrazole 2, 4-(1-adamantyl)pyrazole 3, 3,5-dimethyl-4-(1-adamantyl)pyrazole 4 and 3,5-di(1-adamantyl)pyrazole 5. Compound 2 (a 1∶1 mixture of both tautomers) is a long chain of hydrogen bonded molecules (‘catemer’) and as in most catemers there is no proton transfer since it would imply an ‘infinite’ number of proton jumps. Compound 3, although also a ‘catemer’, is possibly an exception to this rule, in that it seems to show proton transfer. In the solid state, compounds 4 and 5 should be cyclic hydrogen-bonded structures, dimers or trimers, but the activation energies for proton transfer, about 39 kJ mol -1 , are quite low compared with those of 3,5-dimethylpyrazole. It appears that the quasi-spherical shape of the 1-adamantyl substituent and its solid-state plasticity may play a role in lowering these barriers. The crystal structure of 2 has been determined by X-ray analysis. Individual molecules of 2 form chains through N–H · · · N hydrogen bonds (‘catemers’) very similar to those already described for 4-(1-adamantyl)pyrazole and for pyrazole itself; however, the packing of these catemers is different. Tautomers 2a and 2b are present in the crystal in a 1∶1 ratio, forming alternating chains of hydrogen-bonded molecules (2a · · · 2b · · · 2a · · · 2b · · ·); the NH hydrogen atoms are linked to both nitrogen positions (N1 and N2) and show a 1∶1 disorder.

Theoretical characterization of the hydrogen-bond interaction of diacetamide with water and methanol

M. T. Nguyen, N. Leroux and T. Zeegers-Huyskens, J. Chem. Soc., Faraday Trans., 1997, 93, 33 DOI: 10.1039/A602453J

Hydrogen bonding in transient bifunctional hypervalent radicals by neutralization—reionization mass spectrometry

Neutralization-reionization mass spectrometry is used to generate hypervalent 9-N-4 (ammonium) and 9-O-3 (oxonium) radicals derived from protonated α,ω-bis-(dimethylamino)alkanes and α,ω-dimethoxyalkanes, which exist as cyclic hydrogen-bonded structures in the gas phase. Collisional neutralization with dimethyl disulfide, trimethylamine, and xenon of the hydrogen-bonded onium cations followed by reionization with oxygen results in complete dissociation. Bond cleavages at the hypervalent nitrogen atoms are found to follow the order CH2-N>CH3-N>N-H, which differs from that in the monofunctional hydrogen-n-heptyldimethylammonium radical, which gives CH2-N>N-H>CH3-N. No overall stabilization through hydrogen bonding of the bifunctional hypervalent ammonium and oxonium radicals is observed. Subtle effects of ring size are found that tend to stabilize large ring structures and are attributed to intramolecular hydrogen bonding.

PH-dissociation characteristics of cardiolipin and its 2'-deoxy analogue.

Cardiolipin (CL) is found in inner mitochondrial membranes and the plasma membrane of aerobic prokaryotes. CL is tightly bound to those transmembrane enzymes associated with oxidative phosphorylation. CL has earlier been reported to have a single pK at low pH. We have titrated CL in aqueous suspension (bilayers) and in solution in methanol/water (1:1, vol/vol) and found it to display two different pK values, pK1 at 2.8 and pK2 initially at 7.5 but shifting upwards to 9.5 as the titration proceeds. The unusually high pK2 might be explained by the formation of a unique hydrogen bond in which the free hydroxyl on the central glycerol forms a cyclic intramolecular hydrogen-bonded structure with one protonated phosphate (P-OH group). We have therefore chemically synthesized the 2'-deoxycardiolipin analogue, which lacks the central free hydroxyl group, and measured its pH-dissociation behavior by potentiometric titration, under the same conditions as those for CL. The absence of the hydroxyl group changes the titration dramatically so that the deoxy analogue displays two closely spaced low pK values (pK1 = 1.8; pK2 = 4.0). The anomalous titration behavior of the second dissociation constant of CL may be attributed to the participation of the central glycerol OH group in stabilizing the formation of a cyclic hydrogen-bonded monoprotonated form of CL, which may function as a reservoir of protons at relatively high pH. This function may have an important bearing on proton pumping in biological membranes.

Ab initio molecular orbital calculations of the infrared spectra of hydrogen-bonded complexes of water, ammonia and hydroxylamine: Part 7. The water-hydroxylamine complex

The structure, hydrogen-bond energy and IR wavenumbers and band intensities of a 1:1 molecular complex formed between water and hydroxylamine were determined by means of a series of ab initio molecular orbital calculations at the level of second-order Møller-Plesset perturbation theory, using the 6-31G ∗∗ basis set. The optimized geometry is found to be that of a five-membered cyclic hydrogen-bonded structure of C 1 symmetry, containing both OH·O and OH·N interactions. The computed interaction energy is determined to be larger than those of the cyclic water and ammonia homodimers, determined at the same level of theory. The computed IR spectrum is compared with those of the isolated monomers, and the wavenumber shirts and intensity changes are rationalized in terms of hydrogen-bonding theory.

The mechanics of hydrogen bond formation in conjugated systems

The theory of atoms in molecules provides an atomic view of the mechanics governing the interactions between molecules and as such it facilitates both an understanding of the interactions and the manner in which they can be modified. This theory is applied to a study of the changes in the energies, electron populations, volumes and moments of the atoms of formamide upon dimerization to form both the cyclic and open hydrogen-bonded structures. An important factor governing the changes in the relative stabilities of the atoms on forming the dimers are the opposing flows of σ and π density through conjugated NCO fragments, as measured by the shift in the electron populations into or out of the plane of the nuclei over each of the atomic basins and by the changes in the ellipticities of the atomic distributions.

ChemInform Abstract: STRUCTURE, HYDROGEN BONDING AND VIBRATIONAL SPECTRA OF PYRUVIC ACID

Abstract The complete vibrational spectra of liquid pyruvic acid and the infrared spectrum of crystalline pyruvic acid at about 20 K have been recorded and analyzed. A vibrational assignment is proposed based on these spectra and comparison with spectra of derivatives of pyruvic acid. The spectra of pyruvic acid can best be interpreted in terms of a cyclic hydrogen-bonded dimer structure in which the two carbonyl groups are in a trans configuration in the pure liquid phase. A similar structure has been reported for crystalline pyruvic acid by X-ray diffraction. In dilute solution the structure appears to be monomeric with an internal hydrogen bond, in essential agreement with the structures of the monomer reported from microwave spectroscopic measurements.