Hydrazine (N2H4) adsorption on Ni(1 0 0) – Density functional theory investigation

A theoretical study on the structure and adsorption mechanism of hydrazine (N₂H₄) on Ni(1 0 0) are presented. The hydrazine molecule was found to adsorb on the surface through one of its nitrogen atom in its anti-conformation. The charge transfer from hydrazine lone pair orbitals played a key role in the formation of the bonding. The mechanism involved in the bonding was found to reduce the necessity of hyper-conjugation interaction, that reduces the gauche effect found in hydrazine at the gas-phase. Upon adsorption to the surface, the reduced interaction resulted in the promotion of a more favored conformation through its anti-conformation.     

Theoretical study of hydrazine adsorption on Pt(111): Anti or cis?

Hydrazine (N₂H₄) adsorption on metal surface is important due to its application in the direct hydrazine fuel cell technology. First principles DFT calculations have been carried out to understand the structure and mechanism of hydrazine adsorption on Pt(111). Calculations revealed that configuration with hydrazine adsorbed on its anti-conformation yields the largest adsorption energy suggesting it to be the most stable structure on Pt(111). This result was found to be in disagreement with available XPS results which favor the adsorption on cis-conformation as the most stable configuration. However, by taking into account the energy cost for orbital re-hybridization and internal rotation involves in the adsorption, it was found that the interaction strength between adsorbate and substrate is comparably equal for adsorption on both anti and cis-conformations that indicates the feasibility of the adsorption in cis-conformation to occur. Charge transfers from lone-pair orbitals belong to the highest occupied molecular orbital (HOMO) and second highest occupied molecular orbital (S-HOMO) were found to be important in the formation of the bonding. The π-anti-bonding HOMO lone-pair transfers its charge to the surface which stabilizes the internal structure of the molecule and responsible for the stable anti-conformation adsorption structure. The interaction of the π-bonding S-HOMO lone pair with the surface was found to be dative type and plays an important role in the stabilization of cis-conformation adsorption structure.     

Adsorption of hydrazine and ammonia on aluminium

The adsorption and decomposition of hydrazine and ammonia by clean polycrystalline aluminium surfaces have been studied by X-ray photoelectron spectroscopy. At 85 K both ammonia and hydrazine are adsorbed molecularly, with N(1s) peaks at 400.5 eV. At 290 K hydrazine is initially adsorbed to give an N(1s) peak at about 399 eV, but with time (and further exposure) the position of the peak maximum drift to lower N(1s) values, finally approaching 397 eV after heating the ad-layer to 390 K. These observations are interpreted in terms of a slow dissociative chemisorption process: N₂H₄(a) → NH₂(a) → NH(a) → N(a). There is no doubt that the NN bond in hydrazine is broken and that hydrogen ad-atoms formed inhibit the subsequent adsorption of N₂H₄ at 290 K. Ammonia dissociates more slowly than hydrazine to give mainly amine (NH₂) species at 290 K.     

Photoelektronenspektren organischer verbindungen: III. Photoelektronenspektren acetylensubstituierter kleiner ringe

The high resolution He I photoelectron spectra of vinylacetylene, cyclopropylacetylene, cyclobutylacetylene and cyclohexylacetylene are reported and ionization potentials in the low-binding region are assigned by correlating the spectra with spectra of reference compounds, by using the information supplied by vibrational analysis and by comparing the assignment with the results of MINDO/2 calculations. The resonance integrals β_RS between the π type orbitals of the acetylene moiety (R) and the π and Walsh type orbitals of the substituents (S) are determined by means of a LCMO model, parametrized with spectra of known assignment. The interaction terms H_Rs of cyclopropylacetylene, which are 1.3 eV in the bisected conformation and 0.7 eV in the perpendicular conformation, can be considered as experimental verification of the correlation diagram for vinylcyclopropane given by Hoffmann et al. and provide an explanation of the stability of the gauche conformation of vinylcyclopropane. In the case of cyclobutylacetylene, only estimated values of the interaction term H_RS can be obtained from the spectrum.     

A rotational Hamiltonian for the ground vibrational state of hydrazine

A Hamiltonian matrix suitable for fitting rotational energy levels of the hydrazine molecule in its ground vibrational and electronic states was obtained. This matrix is of dimension 16 × 16, where the 16 functions labeling the rows and columns consist of the two members of a near-prolate asymmetric rotor doublet (with given | K_a |) for the eight different, but chemically equivalent, conformers which the molecule can reach by various combinations of -NH₂ inversion at either end of the molecule, and internal rotation about the NN bond. The matrix is derived in a phenomenological fashion, by applying group theoretical arguments to a model in which tunneling among the various frameworks is assumed to be very slow compared with the vibrational frequencies. A comprehensive treatment of the large-amplitude vibrational potential surfaces and associated tunneling pathways has not been carried out, nor have quartic (J⁴) centrifugal distortion effects been considered in a systematic fashion. Preliminary fits indicate that the model developed can be used to fit the hydrazine microwave data in a consistent fashion, and a full treatment of such data has been undertaken.     

离子液体阳离子在空气/[bmim][BF4]水溶液界面的取向和结构

采用和频振动光谱研究了空气/[bmim][BF₄]低浓度水溶液界面的取向结构. 研究发现,在体相浓度非常低时,丁基链具有较大的旁式扭曲,表明此时的取向比较无序;阳离子咪唑环则采取一个较小的取向角. 随着浓度的升高,阳离子咪唑环趋向平躺在界面. 由于链-链相互作用,此时丁基链的旁式扭曲也减小,说明界面分子的取向变得有序. 进一步研究发现,PPP和SPS光谱上甲基反对称的峰存在位移,表明界面丁基链上的甲基存在不同取向或具有不同的化学环境.结果有助于从微观层次理解水溶性离子液体和基于咪唑的表面活性剂在界面上的物理化学行为.     

Conformation-selective adsorption of 2,3-butanediol on Si(0 0 1)

The adsorption of 2,3-butanediol on the Si(0 0 1) surface is studied by means of first-principles pseudopotential calculations. Molecular adsorption on top of the Si dimers resulting in a 6-membered ring of the O-C-C-O segment with the dimer atoms is energetically favored, in agreement with the interpretation of recent experiments. The adsorption energy difference for butanediol adsorbed in either gauche or anti conformation is nearly one order of magnitude larger than the energy difference between the respective conformers in gas phase, pointing to a conformation-selective adsorption.     

Infrared spectra and rotational isomerism of ethylenediammonium ion in aqueous solution

The infrared spectra of ethylenediamine hydrochloride, sulfate and their N-deuterium derivatives in water or heavy water were studied at various pH or pD and at various concentrations. The frequencies of the CH₂ deformation vibration bands in the region 1500-1250 dm^?1 were found to be sensitive to the conformation of the ethlenediamine molecule and mono- and diammonium ions. The behaviors of these bands indicate that in aqueous solution the ethylenediamine molecule takes the gauche form, the ethylenemonoamine monoammonium ion takes mostly the gauche form and the ethylenediammonium ion takes both the trans and gauche forms.     

RAIRS investigations on the orientation and intermolecular interactions of adenine on Cu(1 1 0)

The adsorption of the DNA base adenine (C₅N₅H₅) on Cu(1 1 0) has been investigated as a function of coverage and temperature using reflection absorption infrared spectroscopy. These data provide important information on the nature of the local adsorbed complex and the intermolecular lateral interactions that come into play at high coverages. The RAIR spectra are consistent with an adsorption geometry in which the molecular ring is substantially tilted from the surface plane and in which the two amino hydrogens are equidistant from the surface, thus rationalising the appearance of a very strong βNH₂ scissoring mode, along with the activity of in-plane vibrational modes and the observation of the symmetric ν_symNH₂ stretch, but not the asymmetric ν_asymNH₂ stretch. In addition, coordination to the metal surface is proposed to occur at the N(9) position with a possible additional interaction through the N(3) position, both of which are the favoured coordination points in the metal complexes of adenine. This is a strong interaction and leads to a highly stable adsorbed layer. Our data also provide the first direct evidence of hydrogen bonding in the adlayer as coverage is increased, attributed to interactions between the amino group of one molecule and the N(1) and N(7) positions of a neighbouring species. When adsorption is carried out at room temperature, a very heterogeneous adlayer is created in which a diversity of molecular aggregates co-exist. However, upon annealing, a more ordered hydrogen bonded adlayer is formed in which one type of hydrogen bonding assembly is preferred. Finally, we propose that the hydrogen bonded assemblies created at a surface probably involve bent hydrogen bonds which arise from a compromise between the strong molecule-metal interactions that orientate the molecule and weaker lateral hydrogen bonding interactions that dictate the two-dimensional architecture.     

The microwave spectrum of difluoroacetic acid

Transitions due to two conformations have been observed in the microwave spectrum of difluoroacetic acid. In both rotamers the carboxylic hydrogen atom is cis to the CO bond, whereas the other hydrogen atom is either trans or gauche with respect to the CO bond. By means of isotopic substitution of the hydrogen and oxygen atoms a serious deviation of local symmetry in the CHF₂ group is deduced for the gauche form. This is thought to be an artifact of the substitution method, although no satisfactory explanation can be given. In torsionally excited states of the gauche conformation, doublet splittings were observed due to tunneling through a barrier of 190 ± 10 cm^?1. For the second excited state a satisfactory treatment could be given, but for the third excited state large differences between observed and calculated transition frequencies remained. A computer program to account for the internal rotation in asymmetric molecules was written, and it was shown qualitatively that interactions with excited states of the trans conformation could very well be responsible for the discrepancies. From intensity measurements the energy difference E(gauche) - E(trans) was found to be 370 ± 180 cm^?1. The large uncertainty is due to the failure of attempts to determine the dipole moments experimentally; they were estimated from an improved set of bond moments which was derived from related compounds. Both energy difference and dihedral angles are at variance with the results of an earlier electron diffraction investigation.     

Microwave spectrum and conformation of vinyl mercaptan: The anti rotamer

Microwave spectra of the anti rotamer of vinyl mercaptan and its SD isotopic species have been studied in the frequency range 12–60 GHz. For the normal species rotational and centrifugal distortion constants have been obtained for the ground and first three excited states of the SH torsional mode, the ground state values being A = 49 422.75(5) MHz, B = 5 897.215(9) MHz, C = 5 279.436(9) MHz, D_J = 3.12(11) kHz, D_JK = ?38.50(1.71) kHz, and δ_J = 0.498(51) kHz. An approximate potential function for the SH torsion in the vicinity of the anti conformation, derived using the observed variation of rotational constants with vibrational quantum number, reveals the presence of a small potential barrier of 19 cm^?1 at the planar conformation. The v = 0 state lies above this barrier so the molecule is essentially planar in the ground state in spite of the observed negative value for the inertia defect (?0.1976(2) a.m.u.Ų). The anti rotamer is found to be 50 ± 25 cm^?1 less stable than the syn rotamer. The dipole moment has the ground state values μ_a = 0.425(10), μ_b = 1.033(10), and μ_total = 1.117(14) D and is shown to vary considerably with vibrational quantum number. Evidence for significant structural changes in going from the syn rotamer to the anti rotamer is also presented.     

A DFT study of carbon monoxide adsorption on a Si4 nano-cluster

Using the gradient-corrected hybrid density functional method of Predew, Burke, and Ernzerhof (PBEPBE) and the new hybrid meta-density functional method of Truhlar (MPW1B95), the geometry, adsorption energy, vibrational frequency, and charge distribution of carbon monoxide adsorption on a Si₄ nano-cluster has been studied. Taking into account spin multicipility in the calculations, a new stable structure of CO absorbed on the Si₄ cluster has been found, in addition to the previously reported structures. Exhaustive vibrational frequency analysis of optimized structures shows that some of the formerly reported structures have imaginary vibrational frequencies and are not proper stable structures. Thus, they do not represent real local energy minima. Also, CO vibrational frequency analysis shows that a significant change of vibrational frequency in the stable structures occurs.     

Influence of dietary fibre on gluten proteins structure – a study on model flour with application of FT‐Raman spectroscopy

Dietary fibres are regarded as the source of polysaccharides and antioxidants such as polyphenols. However, addition of dietary fibre to bread causes significant reduction in its quality. The bread quality is connected with the structure of gluten proteins. For this reason, Fourier transform Raman spectroscopy was applied to determine changes in structure of gluten proteins modified by seven dietary fibres. The fibres were added to model flour reconstituted with wheat gluten and wheat starch. The model flour was used to provide gluten proteins of definite structure. The obtained results showed that six out of seven fibres caused similar changes in β‐turn structures. The appearance of the band at 1642 cm⁻¹ and the shift toward lower wavenumbers of the band at 1670 cm⁻¹ in the difference spectra indicated hydrogen bonding of carbonyl groups in β‐turns leading to protein folding/aggregation. Addition of fibre preparations caused also changes in conformation of disulfide bridges (S–S), corresponding to transformation to trans‐gauche‐gauche and trans‐gauche‐trans conformations at the expense of the stable gauche‐gauche‐gauche conformation. The S–S bonds in less stable conformations were formed inside the protein complex as well as between protein complexes in the form of β‐structures. Generally, the observed changes in gluten proteins after addition of dietary fibres were results of interactions between fibre polysaccharides and gluten proteins rather than between polyphenols and gluten proteins. Copyright © 2015 John Wiley & Sons, Ltd.     

半胱胺单分子膜结构的表面增强拉曼散射研究(英文)

在金属电极表面所形成的有机分子的单分子膜或薄膜对于基础研究和实际应用都有着极其重要的意义。以化学吸附形式在金电极表面所形成的半胱胺单分子膜,常常用于蛋白质等生物大分子在金属表面进行吸附的连接层,以避免这些生物大分子在金属表面直接吸附而造成的变性、失活现象的发生。本文报道了我们采用表面增强拉曼散射光谱方法研究在金电极表面吸附的半胱胺单分子膜的结构特征。研究结果表明,在金电极表面所形成的半胱胺单分子膜中,半胱胺分子主要的构型为扭转构型。在与金表面的相互作用中,由于除巯基的结合作用以外,还存在半胱胺分子中端基氨基和金表面较强的亲和性,使得以扭转构型吸附在金表面的半胱胺单分子膜相当稳定。这是金电极表面的半胱胺单分子膜结构的主要行征。当考察外加电势对此单分子膜结构的影响时,可以发现有关扭转构型的特征谱峰其相对强度随着电位负移而减小。这一结构随电位的变化关系可以通过表面电势的变化对氨基氮原子上孤对电子与金属表面间相互作用的影响来加以阐释。     

动力学研究AlN/α-Al2Ｏ3（0001）薄膜生长初期的吸附与扩散

采用基于第一性原理的从头计算分子动力学方法,计算了300—800℃下AlN吸附过程与系统能量、动力学轨迹以及扩散系数.研究表明,吸附过程由物理吸附、化学吸附和表面稳定态三个阶段组成,在吸附成键过程中,温度越高,粒子平均表面扩散能力增强.N原子的扩散系数大于Al原子的扩散系数,尤其是在物理吸附阶段.在较高温度条件下（大于700℃）,N的解吸附作用明显增强,不利于AlN的稳定吸附生长,500—700℃之间的温度有利于AlN在α-Al₂Ｏ₃（0001）表面的稳定吸附生 关键词： 2Ｏ₃（0001）表面')" href="#">α-Al₂Ｏ₃（0001）表面 扩散 吸附生长 从头计算分子动力学     

Microwave spectrum,dipole moment,and structure of 3-aminopropanol

The microwave spectra of 3-aminopropanol and three of its deuterium substituted isotopic species have been investigated in the 26.5 to 40 GHz frequency region. The rotational spectrum of only one conformer has been assigned in which presumably a hydrogen bond of the OH---N type exists. The rotational spectra of a number of excited vibrational states have been observed and assignments made for some of these excited states. The average intensity ratio for the rotational transitions between the ground and excited vibrational states indicates that the first excited state is about 120 cm^?1 above the ground state.and the next higher state is roughly 200 cm^?1 above the ground vibrational state. The dipole moment was determined from the Stark effect measurements to be 3.13 ± 0.04 D with its principal axes components as |μ_a| = 2.88 ± 0.03 D, |μ_b| = 1.23 ± 0.04 D and |μ_c| = 0.06 ± 0.01 D. The possibility of another conformer where the hydrogen bond could be of NH---O type was explored, but the spectra of such a conformer could not be identified.     

HCNO as a semirigid bender: The degenerate ν4 state

The semirigid bender Hamiltonian for fulminic acid HCNO (Bunker, Landsberg, and Winnewisser, J. Mol. Spectrosc.74, 9–25 (1979)) is extended. The extended Hamiltonian describes the manifold of large amplitude vibrational states (due to the ν₅ HCN bending mode) superimposed on a high frequency vibrational state involving excited quanta of the ν₄ CNO bending mode. Such high frequency vibrational states may be degenerate when the large amplitude coordinate is zero, and the semirigid bender Hamiltonian is modified to account for the ν₄ vibrational angular momentum around the molecular axis in the linear limit, and for l-doubling effects. The extended Hamiltonian is used to fit HCN bending and rotation energy level separations for HCNO superimposed in the ν₄ fundamental level. It is found that the effective HCN bending potential in the ν₄ state is very similar to that in the high frequency vibrational ground state. The results obtained confirm the conclusion reached by Bunker, Landsberg, and Winnewisser: HCNO is linear at equilibrium.     

Evidence of the weakness of the OH⋯F hydrogen bond from a conformational study of 3-fluoro-1-propanol by microwave spectroscopy

The rotational spectra of the OH and OD isotopic species have been observed for three rotamers of 3-fluoro-1-propanol. One of them (HBC form) displays an internal hydrogen bond with a distorted chair conformation of the six-membered ring. The other two rotamers have the oxygen atom gauche with respect to the C₂C₃ bond, the hydroxyl hydrogen trans with respect to the C₁C₂ bond and the fluorine atom gauche (GGT form) and trans (TGT form), respectively, with respect to the C₂C₁ bond. The energies of the vibrational ground states of the HBC and TGT forms are ～0.4 and 1.0 kcal/mole higher than that of the GGT form, respectively (from relative intensity measurements). The hydrogen bond is therefore rather weak in this compound. With compounds capable of forming OH?O or OH?N bonds, the conformation appropriate for hydrogen bonding is normally the most stable form. Several excited states have been analyzed for the TGT and GGT rotamers in order to have additional data with respect to the potential function for the internal rotation about the C₃C₂ bond.     

Microwave and Infrared Spectra,ab InitioCalculation, and Two-Dimensional Model of Amino Group Inversion and Ring Puckering in 2,5-Dihydropyrrole

The microwave spectra of 2,5-dihydropyrrole and 2,5-dihydropyrrole-1-d₁have been measured with Stark and Fourier transform spectrometers in the range 10–39 GHz. Rotational constants, centrifugal distortion constants, and¹⁴N quadrupole coupling constants have been determined from the observed transition frequencies for the ground vibrational state. In addition, two satellites of the normal species and one satellite of the deuterated species have been identified and measured. Splittings of the rotational transitions due to amino group inversion tunneling have been observed and analyzed. Infrared transitions of the amino group inversion mode have been measured in the range 490–720 cm⁻¹. The effect of ring puckering on the inversion motion of the amino group in 2,5-dihydropyrrole and 2,5-dihydropyrrole-N-d₁has been investigated byab initiocalculations and two-dimensional flexible model calculations from the results of microwave and infrared spectroscopy. The observed molecular properties have been reproduced by a model which involved adjustable parameters for the potential energy surface and the structural relaxation of the CCC valence angles. Additional parameters have been transferred from theab initiocalculations. The adjustment of the model to the experimental data has yielded an equatorial equilibrium conformation with slightly larger CCC valence angle than in the most stable axial conformation. Excitation of the first ring puckering state has been found to enhance the inversion tunnel splittings.     

Dimeric form of peroxynitrite

The (ONOO)^? anion, known as peroxynitrite, is characterized by a singlet spin state. A new stable dimer form of peroxynitrite [NO-O₂]^? in the triplet state at distances close to r(O-N) ≈ 2.885 Å between oxygen in the O₂ structure and nitrogen has been established within the quantum-chemical CASSCF approximation. The vibrational motion of the dimer is significantly anharmonic; for the ¹⁶O and ¹⁴N isotopes, the differences in the energies of two neighboring levels correspond to frequencies of about 70–30 cm^?1. The triplet dimer structure retains stability in the case of interaction with water molecules. The activation barriers caused by rearrangement of the peroxynitrite structure into the ground state of the NO ₃ ^? anion with the symmetry D _3h are found.