Hydrogen-bonded NH Research Articles

Organic salts formed by 2,4,6-triaminopyrimidine and selected carboxylic acids via a variety of hydrogen bonds: Synthons cooperation, and crystal structures

By using solvent evaporation method, 2,4,6-triaminopyrimidine (TAPI) is employed to crystallize with a variety of acids, including 3,5-dihydroxybenzoic acid (HDHBA), 3-nitrophthalic acid (H2NPA), 5-amino-2,4,6-triiodoisophthalic acid (H2ATIPIA), 2,5-dibromoterephthalic acid (H2DBTPA), 1,5-naphthalenedisulfonic acid (H2NDSA), sebacic acid (H2SA), 1,2,4-benzenetricarboxylic acid (H3BTA), and biphenyl-2,2′,5,5'-tetracarboxylic acid (H4BPTA). In all eight complexes, protons are completely exchanged from O atom of acid to nitrogen of TAPI in 1, 3, 4, and, 5, partly transferred in 2, 6, 7, and 8. The crystal structure of all eight complexes exhibit that classical robust hydrogen bonds X-H⋯X (X = O/N) direct the molecular crystals to bind together in a stacking modes. Classical hydrogen bond NH⋯O is participated in forming all eight organic salts, while hydrogen bonding OH⋯O are found in constructing the diversity structures in salts 1, 2, 3, 4, 6, and 7. The analysis shows that some classical supramolecular synthons, such as I R22(8), V R24(12), and VI S(6), are observed again in the construction of hydrogen-bonding networks. In the formation of layered and reticular structure, strong hydrogen bonds between water molecules and ligands having well-refined hydrogen atoms have been considered. Water molecules play an important role in building supramolecular structures of 1, 2, 3, 4, 7, and 8. Moreover, salts 1-8 are further characterized and analyzed by element analysis, infrared radiation, thermogravimetric analysis, proton nuclear magnetic resonance spectra, and mass spectra.

Characterization and coordination of intercalation - Non-intercalation phase transition in a new hybrid halide perovskite (n-C16H33NH3)2 ZnCl4

The ac conductivity σ(ω,T) and the complex dielectric permittivity ε•(ω,T) as function of temperature 100 K < T < 400 K and frequency in the range 1 kHz ≤ f ≤ 100 kHz, as well as some thermal characterizations, namely the differential scanning calorimetry (DSC) and the differential thermal analysis (DTA) thermograms, were, measured and presented for polycrystalline virgin samples of the titled compound. It is one member of the long-chain of Zn hybrids of the series (n-CnH2n+1NH3)2ZnCl4 (n = 8–18) and hence it belongs to the class of thermal energy storage materials.Our data evidenced that the hybrid undergoes a first order solid-solid structural phase transition from an intercalation to a non-intercalation at ≈373 K which is associated with an increase in the interlayer spacing “d” by 19.3%. In the room temperature phase, the hybrid is intercalated and tends to crystallize in an isolated square tetrahedra of zero-dimensional (0D) but arranged in the crystal system in two-dimensional (2D) via the assisted charge hydrogen bonds NH…Cl. It is therefore highly packed and hence have a highly transition temperature, highly thermal energy storage compared with other hybrids of the same n but of different M. These novelties characteristics suggested that the hybrid is more suitable for solar energy research. The even-odd effect, which is only common for Zn and Co in the room temperature phase and absent for other compounds, as well as the mechanism of the phase change have been discussed. The role of the coordination of the divalent metal ion and the dimensionality of the isolated unit (MCl4)−2 (M = Zn and/or Co) are considered. Comparison of the current phase transition with that of Zn compounds (n = 8, 10, 12, and 14) has been given.

Hydrogen Bonding of 1,2-Azaborines in the Binding Cavity of T4 Lysozyme Mutants: Structures and Thermodynamics.

Protein crystallography and calorimetry were used to characterize the binding of 1,2-azaborines to model cavities in T4 lysozyme in direct comparison to their carbonaceous counterparts. In the apolar L99A cavity, affinity for Ab dropped only slightly versus benzene. In the cavity designed to accommodate a single hydrogen bond (L99A/M102Q), Gln102═O···H-N hydrogen bonding for Ab and BEtAb was observed in the crystallographic complexes. The strength of the hydrogen bonding was estimated as 0.94 and 0.64 kcal/mol for Ab and BEtAb, respectively. This work unambiguously demonstrates that 1,2-azaborines can be readily accommodated in classic aryl recognition pockets and establishes one of 1,2-azaborine's distinguishing features from its carbonaceous isostere benzene: its ability to serve as an NH hydrogen bond donor in a biological setting.

A theoretical study of intramolecular H–bonding and metal–ligand interactions in some complexes with bicyclic guanidine ligands

The reported complexes formulated as [MnCl2(hppH)2] (1), [FeCl2(hppH)2] (2), and [NiCl2(hppH)2] (3) and a new theoretically designed example, [CuCl2(hppH)2] (4), have been used for calculations at the B3LYP/LANL2DZ/6-311G (d, p) level of density functional theory (DFT). The intramolecular hydrogen bonds (HB) NH⋯Cl could followed through their physicochemical properties such as, vibrational frequency, electronic transmission in UV–Visible spectroscopy, metal–ligand donor–acceptor interactions in NBO analysis, total energy and frontier molecular orbital energy. These properties influenced by the relationship of structure and metal–ligand electron density exchange will be discussed. The computations revealed that the stronger NH⋯Cl HB exists in complexes with longer MCl and MNimine bonds and shorter H⋯Cl bond, and vice versa that confirms the shortest and longest HBs in 4 and 3, respectively. These results agree with the second–order perturbation energies obtained by NBO analysis within charge transfer from the proton–acceptor chlorine to the p∗ orbital of the N atom. The calculated electronic absorption spectra by TD–DFT calculations show the larger Cl− to M2+ donation in 2 and 3 in comparison with 1 and 4 that confirms the stronger HB in 1 and 4 compared to 2 and 3. In order to find the basis set effect on the structure, vibrational frequencies, and electronic transitions, we use another basis set def2-TZVP includes polarization function for Mn element in complex 1. The obtained test results showed unremarkable differences between two basis sets LANL2DZ and def2-TZVP (cf. Figs. S2, S3 and Tables S1 and S2 in Supplementary Materials).

Double hydrogen bond interaction in 7-azaindole complexes with protic solvents

The double hydrogen bond interaction between 7-azaindole (7AI) and protic solvents including methanol (MeOH), formamide (FM), formic acid (FA), pyridone (PY) and 7AI has been investigated as a proper model of mutations generated by tautomeric shifts in hydrogen-bonded bases of DNA. The results confirm electrostatic nature for all considered hydrogen bonds except for NH hydrogen bond in 7AI-FA. The largest interaction and polarization energies are obtained for 7AI-FA by means of energy decomposition analyses (EDA). The EDA results show an inverse competitive correlation between polarization and electrostatic components of attraction energy to determine the nature of hydrogen bond. The red-shifted hydrogen bonds are identified for all complexes as a result of hyperconjugation, except for 7AI-MeOH, which its blue-shift effect is attributed to the rehybridization dominating of CH bond. Investigation of aromaticity indices for 7AI complexes represents an increase in aromaticity of pentagonal ring and a decrease in hexagonal ring. Finally, the double hydrogen bond between 7AI and FA is identified as dominant interaction to achieve the tautomerization of 7AI in all applied approaches.

Pressure-induced preference for solvation of 5,6-dimethylbenzimidazole

5,6-Dimethylbenzimidazole (C9H9N2) crystallizes exclusively in the unsolvated form from an aqueous solution up to 0.25 GPa and from methanol, ethanol or their mixed solutions up to 0.6 GPa. At still higher pressures, solvated crystals with water, methanol and ethanol are formed depending on the solvent used for recrystallization. The NH⋯N bonds of the neat crystals are replaced by NH⋯OH⋯N bonds in the solvates. This preference for high-pressure solvation has been rationalized in terms of changed hydrogen bonds as well as volume balance between neat and solvated compounds. The methanol solvate C9H9N2·MeOH undergoes a pressure-induced phase transition at 1.40 GPa, lowering its symmetry from monoclinic space group P21/c (phase α) to triclinic space group P (high-pressure phase β) and ordering the H-atoms in hydrogen bonds NH⋯OH⋯N, disordered in phase α.

Self-Assembly of a Tris(Urea) Receptor as Tetrahedral Cage for the Encapsulation of a Discrete Tetrameric Mixed Phosphate Cluster (H2PO4–•HPO42–)2

A tris(2-aminoethyl)amine-based tris(urea) receptor L, in the presence of excess TBA(H2PO4) self-assembled into a tetrahedral molecular cage by encapsulation of a tetrameric tetrahedral mixed phosphate cluster (H2PO4–•HPO42–)2 via 24 −NH hydrogen bonds [complex (n-TBA)6{4L(H2PO4–•HPO42–)2}] (1). Further, in the presence of orthophosphoric acid the receptor L has been observed to be self-assembled with side-cleft bonded tetrameric anion-acid cluster (H2PO4–•H3PO4)2 [complex {(LH)+(H2PO4–•H3PO4)DMSO•H 2O}] (2). In a proof of concept experiment, transformation of salt complex 2 into neutral phosphate encapsulated complex has been demonstrated in solution state by quantitative 1H NMR titration with (TBA)OH in DMSO-d6 (298 K). Extensive studies with receptor L have also been carried out with other halides in the solid state. Crystal structure elucidation revealed the formation of a pseudodimeric (2 + 2) capsular self-assembly with 1:1 host:guest ratio in fluoride encapsulated complex, (TEA)[L(F)] (3). 2D-NOESY NMR (DMSO-d6) experiments were carried out on the phosphate complexes (complexes 1 and 2) to study the binding discrepancies of phosphate anion with the neutral and charged host.

Steady-State Spectroscopy of the 2-(N-methylacetimidoyl)-1-naphthol Molecule.

The steady-state spectroscopy of 2-(N-methylacetimidoyl)-1-naphthol (MAN) reveals composite absorption and emission spectra from 298 to 193 K in hexane. The ground electronic state (So) absorption can be assigned to the sum of three molecular structures: the OH normal tautomer, and two NH proton transfer tautomers. The NH-structures are the most stable ones in equilibrium with the OH tautomer for the S0 state. On photoexcitation of the OH tautomer the excited state intramolecular proton transfer is undergone, and the corresponding NH emission is monitored at 470 nm. On photoexcitation of the NH tautomers the previous emission is monitored in addition to another emission at 600 nm, which is ascribed to intramolecular hydrogen-bonded (IHB) nonplanar NH structures generated from the IHB planar NH tautomers. A Jabłoński diagram is introduced which gathers all the experimental evidence as well as the theoretical calculations executed at the DFT-B3LYP and TD-DFT levels. The MAN molecule is compared with other analogs such as 1-hydroxy-2-acetonaphthone (HAN), 2-(1'-hydroxy-2'-naphthyl)benzimidazole and methyl 1-hydroxy-2-naphthoate to validate the theoretical calculations. Photoexcitation of MAN generates two emission bands at longer wavelengths than that of the emission band of HAN. The MAN molecule exhibits a great photostability in hydrocarbon solution which depends on the photophysics of the NH tautomers (keto forms).

Design and synthesis of biotin analogues reversibly binding with streptavidin.

Two new biotin analogues, biotin carbonate 5 and biotin carbamate 6, have been synthesized. These molecules were designed to reversibly bind with streptavidin by replacing the hydrogen-bond donor NH group(s) of biotin's cyclic urea moiety with oxygen. Biotin carbonate 5 was synthesized from L-arabinose (7), which furnishes the desired stereochemistry at the 3,4-cis-dihydroxy groups, in 11% overall yield (over 10 steps). Synthesis of biotin carbamate 6 was accomplished from L-cysteine-derived chiral aldehyde 33 in 11% overall yield (over 7 steps). Surface plasmon resonance analysis of water-soluble biotin carbonate analogue 46 and biotin carbamate analogue 47 revealed that KD values of these compounds for binding to streptavidin were 6.7×10(-6) M and 1.7×10(-10) M, respectively. These values were remarkably greater than that of biotin (KD =10(-15) M), and thus indicate the importance of the nitrogen atoms for the strong binding between biotin and streptavidin.

Solution and solid-state study of the structure of azo-coupling products from isomeric enaminones possessing tert-butyl group: An unprecedented observation of pure hydrazo form in azo coupled N-alkyl β-enaminones

The structure of the azo-coupling products from enaminones derived from 4,4-dimethyl-1-phenylpentane-1,3-dione has been studied by means of solution-state 1H, 13C and 15N NMR spectroscopy and X-ray diffractometry. The presence of bulky tert-butyl group hinders or even prevents from the formation of planar conjugated heterodiene system HNCCNN with an intramolecular hydrogen bond NH⋯N which is the prerequisite for fast tautomeric exchange imino-hydrazo – enamino-azo. The minor amount of azo compounds is formed by a proton exchange through a hydrogen bond NH⋯N, which is either intramolecular (in solution) or intermolecular (solid state). The intermolecular exchange proceeds via the dimers of the azo coupling products. This is unprecedented result among the similar molecules hitherto studied.

Conformational structure of N-(silylmethyl)anilines PhNHCH2SiMe n (OEt)3-n (n = 0–3)

As show the data of IR spectroscopy and quantum-chemical calculations (B3LYP/6-311+G**), N-(silylmethyl)anilines PhNHCH2SiMen(OEt)3−n in inert media have an intramolecular hydrogen bond NH⋯OSi. N-[(Trimethylsilyl)methyl]aniline PhNHCH2SiMe3 in inert solvents exists as a mixture of two conformers close in energy.

Vapour pressures of 1-methyl derivatives of benzimidazole, pyrazole and indole. The energy of the intermolecular hydrogen bond N[sbnd]H⋯N

The vapour pressures of the liquid phase of 1-methylpyrazole, 1-methylbenzimidazole and 1-methylindole were measured over the temperature ranges (253.9 to 293.3)K, (303.2 to 372.5)K, and (268.6 to 341.9)K, respectively, using a static method. The vapour pressures of the crystalline phase of the two latter compounds were also measured at temperatures between (301.2 to 328.9)K and (267.6 to 275.5)K, respectively. The results obtained enabled the determination of the standard molar enthalpies and entropies of sublimation and of vaporisation at the mean temperatures of the measurements and at T=298.15K. The temperatures and molar enthalpies of fusion were determined using differential scanning calorimetry. The enthalpies of the intermolecular hydrogen bonds NH⋯N in the crystalline phase of benzimidazole and pyrazole were determined and compared with the result previously determined for the energy of the intermolecular hydrogen bond in crystalline imidazole.

Interaction of DNA nucleotide bases with anticancer drug ThioTEPA: molecular docking and quantum-mechanical analysis

Using modern methods of molecular docking, quantum chemistry and quantum theory of atoms in molecules the interaction of anticancer drug ThioTEPA with isolated nucleotide bases and deoxyribonucleosidemonophosphates of DNA has been studied. Physical properties and some trends of binding have been established for the complexes of "nucleotide base + ThioTEPA" and "deoxyribonucleosidemonophosphate + ThioTEPA" types. It has been shown that strong hydrogen bonds of NH...N type are the key factor responsible for high selectivity of binding of ThioTEPA to the guanine-containing units of the DNA.

The Complexation of the Anticancer Drug ThioTEPA with Methylated DNA Base Guanine: Combined Ab Initio and QTAIM Investigation.

Non-covalent complexes of methylated nitrogenous DNA base guanine (m(9) Gua) with 1 to 6 molecules of anticancer drug ThioTEPA (1,1',1''-phosphorothioyltriaziridine) have been investigated by molecular modeling techniques (molecular docking and DFT geometry optimization), ab initio wavefunction calculations and the quantum theory of atoms in molecules (QTAIM). The accuracy of complex structures predicted by standard molecular docking techniques have been assessed by comparing them with ab initio calculations, and the most important differences have been discussed. Obtained stabilization enthalpies (kcal/mol) for the m(9) Gua⋅⋅⋅(ThioTEPA)n complexes with n=1…6 have been found to be -15.6, -26.5, -38.4, -49.6, -60.5 and -69.3 respectively. The non-covalent interactions revealed by the QTAIM method have been shown to be a dominating factor responsible for the complex stability, with hydrogen bonds of NH⋅⋅⋅N type being the most important interactions in small (n=1 to 4) and CH⋅⋅⋅N bonds - in large (n=5, 6) complexes. The obtained results may help to understand ThioTEPA-DNA interactions and clarify the mechanism of the drug action.

Preparation, structure and properties of pyridinium/bipyridinium hexafluorosilicates

Pyridinium hexafluorosilicates with the compositions (LH)2[SiF6] (where L=2,6-bis(hydroxymethyl)pyridine (I), 4,5-bis(hydroxymethyl)-2-methylpyridine-3-ol (II)), monohydrate (LH)2[SiF6]·H2O (L=2-bromo-6-methylpyridine (III)) and (LH2)[SiF6] (L=4,4′-bipyridine (IV), 2,2′-bipyridine (V)) were separated as crystalline products of interaction of fluorosilicic acid with different pyridines. All compounds were characterized by elemental analysis, IR, NMR 19F and mass-spectrometry, solubility data, and X-ray crystallography. The structural study revealed the details of the anion binding and solid state supramolecular architectures provided by the combination of the plethora of intermolecular interactions including strong charge assisted and conventional hydrogen bonds of NH⋯F, OH⋯F types along with O⋯Br contacts and π–π interactions. The relationship between the salts structure and physico-chemical properties is discussed.

Structural Diversity of Copper(I) Complexes Formed by Pyrrole- and Dipyrrolylmethane-Based Diphosphine Ligands with Cu–X···HN Hydrogen Bonds

Metal complexes containing hydrogen bond donor/acceptor groups are interesting because of their applications in several areas. In the course of our investigation on the synthesis of metal complexes using newly developed pyrrole-based diphosphine ligands, a few structurally interesting copper(I) complexes containing the pyrrolic NH hydrogen bond donors were synthesized. The reaction of 2,5-bis(diphenylphosphinomethyl)pyrrole (PNP) with an equimolar quantity of CuX (X = Cl, Br, and I) afforded the binuclear copper(I) complexes [Cu(μ-X)(μ-PNP-P,P)]2 (1-3) in very good yields (87-90%). Conversely, the analogous reaction between 1,9-bis(diphenylphosphinomethyl)diphenyldipyrrolylmethane (PNNP) and CuX (X = Cl, Br, and I) yielded the mononuclear Cu(I) complexes [CuX(PNNP-P,P)] (4-6) in very good yields (∼88%), in which the diphosphine ligand is chelated to the copper metal atom. Interestingly, when this reaction was carried out with a 1:2 mol ratio of ligand/metal, the cubane-like tetranuclear Cu(I) complex, [Cu4I4{μ-Ph2C(C4H3N)2-1,9-(CH2PPh2)2-P,P}2] 7, was isolated in 68% yield. In addition, the reaction between the dipyrrolyldiphosphine ligand (PNNP) and CuCl in the presence of 1 equiv of 1,10-phenanthroline monohydrate and NaBF4 afforded a novel ionic binuclear mixed-ligand Cu(I) complex, [Cu2(μ-X)(μ-PNNP-P,P)(NN)2]BF4 8, where NN = 1,10-phenanthroline in 57% yield. The structures of all these complexes were confirmed by the single-crystal X-ray diffraction method and are supported by spectroscopic data. In contrast to the PNP pincer ligand, the dipyrrolyl-diphosphine ligand (PNNP) adopts chelation as well as bridging coordination modes with Cu(I) atoms, indicating its flexibility of bonding. In all the structures, the Cu-X···HN type of hydrogen bonds involving the metal halide ion as acceptor and the pyrrolic NH as donor are present with the Cu-X···H angles, which deviate from the favored 90°, as observed in their solid state structures. Further, the presence of this type of hydrogen bond was confirmed by NBO, AIM, and Hirshfeld analyses.

Preorganized Anion Traps for Exploiting Anion-π Interactions: An Experimental and Computational Study

1,3-Bis(pentafluorophenyl-imino)isoindoline (A(F)) and 3,6-di-tert-butyl-1,8-bis(pentafluorophenyl)-9H-carbazole (B(F)) have been designed as preorganized anion receptors that exploit anion-π interactions, and their ability to bind chloride and bromide in various solvents has been evaluated. Both receptors A(F) and B(F) are neutral but provide a central NH hydrogen bond that directs the halide anion into a preorganized clamp of the two electron-deficient appended arenes. Crystal structures of host-guest complexes of A(F) with DMSO, Cl(-), or Br(-) (A(F):DMSO, A(F):Cl(-), and A(2)(F):Br(-)) reveal that in all cases the guest is located in the cleft between the perfluorinated flaps, but NMR spectroscopy shows a more complex situation in solution because of E,Z/Z,Z isomerism of the host. In the case of the more rigid receptor B(F), Job plots evidence 1:1 complex formation with Cl(-) and Br(-), and association constants up to 960 M(-1) have been determined depending on the solvent. Crystal structures of B(F) and B(F):DMSO visualize the distinct preorganization of the host for anion-π interactions. The reference compounds 1,3-bis(2-pyrimidylimino)isoindoline (A(N)) and 3,6-di-tert-butyl-1,8-diphenyl-9H-carbazole (B(H)), which lack the perfluorinated flaps, do not show any indication of anion binding under the same conditions. A detailed computational analysis of the receptors A(F) and B(F) and their host-guest complexes with Cl(-) or Br(-) was carried out to quantify the interactions in play. Local correlation methods were applied, allowing for a decomposition of the ring-anion interactions. The latter were found to contribute significantly to the stabilization of these complexes (about half of the total energy). Compounds A(F) and B(F) represent rare examples of neutral receptors that are well preorganized for exploiting anion-π interactions, and rare examples of receptors for which the individual contributions to the binding energy have been quantified.

A Study on the Structure of Novel Polyurethanes Derived from γ-Valerolactone-Based Diol Precursors

As versatile biomass-based diol precursors, N,N'-1,2-ethanediylbis-(4-hydroxy-pentanamide) (1) and 4-hydroxy-N-(2-hydroxyethyl)-pentanamide (2) are potential monomers to synthesize novel polyurethanes through adding di-isocyanates. This study reported the structural analysis and molecular behavior of polyurethanes obtained from polymerization of the diol precursors with aliphatic and aromatic di-isocyanates (hexamethylene diisocyanate, HDI (3), and phenyl-diisocyanate, PDI (4)) in (N,N-dimethylacetamide (DMA) solvents with triethylamine (TEA) catalysts. 1H-NMR, 13C-NMR and Elemental Analysis confirmed structure of the polyurethanes built from both diols and di-isocyanates and FTIR indicated interaction among polyurethane molecules showed at lower wave numbers such as 2855-2976 cm-1 for hydrogen-bonded NH groups and 1621-1643 cm-1 for hydrogen-bonded C=O groups. Furthermore a study on influence of the inter-and intra-molecular hydrogen bonding on the thermal and mechanical properties of the polyurethanes would be an interesting investigation for the next study.

A Study on the Structure of Novel Polyurethanes Derived from γ-Valerolactone-Based Diol Precursors

As versatile biomass-based diol precursors, N,N'-1,2-ethanediylbis-(4-hydroxy-pentanamide) (1) and 4-hydroxy-N-(2-hydroxyethyl)-pentanamide (2) are potential monomers to synthesize novel polyurethanes through adding di-isocyanates. This study reported the structural analysis and molecular behavior of polyurethanes obtained from polymerization of the diol precursors with aliphatic and aromatic di-isocyanates (hexamethylene diisocyanate, HDI (3), and phenyl-diisocyanate, PDI (4)) in (N,N-dimethylacetamide (DMA) solvents with triethylamine (TEA) catalysts. 1H-NMR, 13C-NMR and Elemental Analysis confirmed structure of the polyurethanes built from both diols and di-isocyanates and FTIR indicated interaction among polyurethane molecules showed at lower wave numbers such as 2855-2976 cm-1 for hydrogen-bonded NH groups and 1621-1643 cm-1 for hydrogen-bonded C=O groups. Furthermore a study on influence of the inter-and intra-molecular hydrogen bonding on the thermal and mechanical properties of the polyurethanes would be an interesting investigation for the next study.

NH3 as a Strong H-Bond Donor in Singly- and Doubly-Bridged Ammonia Solvent Clusters: 2-Pyridone·(NH3)n, n = 1–3

Mass- and isomer-selected infrared spectra of 2-pyridone·(NH3)n clusters with n = 1-3 were measured in the NH and CH stretch fundamental region (2400-3700 cm(-1)) using infrared (IR) laser depletion spectroscopy combined with resonant two-photon ionization UV laser detection. The IR depletion spectra reveal three different H-bonding topologies of these clusters: The n = 1 and 2 clusters form ammonia bridges stretching from the N-H to the C═O group of the cis-amide function of 2-pyridone (2PY), giving rise to intense and strongly red-shifted (2PY)NH and ammonia NH stretch bands. For n = 3, two isomers (3X and 3Y) are observed in the IR spectra: The spectrum of 3X is compatible with an ammonia-bridge structure like n = 2, with the third NH3 accepting an H-bond from C(6)-H of 2PY. The IR spectrum of 3Y exhibits a broad IR band in the 2500-3000 cm(-1) range and is characteristic of a bifurcated double-bridged structure in which the first NH3 accepts an H-bond from the (2PY)NH and donates two H-bonds to the other two ammonias, both of which donate to the C═O group of 2PY. This double-donor/double-bridge H-bonding pattern increases the acceptor strength of the first ammonia and dramatically lowers the (2PY)NH stretching frequency to ∼2700 cm(-1). For all clusters the ammonia 2ν4 HNH bend overtones in the 3180-3320 cm(-1) region gain intensity by anharmonic coupling (Fermi resonance) to the hydrogen-bonded ammonia NH stretches, which are red-shifted into the 3250-3350 cm(-1) region. The experimental results are supported by optimized structures, vibrational frequencies, and IR intensities calculated using density-functional theory with the B3LYP and PW91 functionals, as well as with the more recent functionals B97-D and M06-2X, which are designed to include long-range dispersive interactions.