Unraveling the origin of band shapes in infrared spectra of N2O-12CO2 and 12CO2-13CO2 ice particles

Band structures in the region of strong infrared absorption bands for different N2O-12CO2 and 12CO2-13CO2 composite particles are investigated by combining quantum mechanical exciton calculations with systematic experimental investigations. The ice particles are generated by collisional cooling and characterized with rapid-scan infrared spectroscopy. The size of the particles lies between approximately 10 and 100 nm. The calculated spectra show excellent agreement with the experimental data. This work leads to a detailed understanding on a molecular level of shape effects in pure and statistically mixed particles as well as of the characteristic features observed for core-shell particles.     

Phase, shape, and architecture of SF6 and SF6/CO2 aerosol particles: infrared spectra and modeling of vibrational excitons

Information on the phase, shape, and architecture of pure SF(6) and mixed SF(6)/CO(2) aerosol particles is extracted from experimental infrared spectra by comparison with predictions from quantum mechanical exciton calculations. The radius of the particles lies around 50 nm. The following extensions to our previous vibrational exciton model are included: (i) To account for the many degrees of freedom of degenerate vibrational bands of aerosol particles, we take a time-dependent approach to calculate infrared absorption spectra directly from the dipole autocorrelation function. (ii) In addition to the dipole-dipole interaction, dipole-induced dipole terms are included to account for the high polarizability of SF(6) and CO(2). We find SF(6) aerosol particles with a cubiclike shape directly after their formation and a change in the shape toward elongated particles with increasing time. Our microscopic model reveals that the cubic-to-monoclinic phase transition at 96 K found in the bulk cannot be observed with infrared spectroscopy because the two phases show almost identical spectra. Infrared spectra of two-component SF(6)/CO(2) particles with core-shell structure show characteristic split absorption bands for the shell. By contrast, homogeneously mixed SF(6)/CO(2) particles lead to broad infrared bands for both the core and the shell. The molecular origin of these various spectral features is uncovered by the analysis of the vibrational eigenfunctions.     

DFT studies of structure and vibrational frequencies of isotopically substituted diamin uranyl nitrate using relativistic effective core potentials

The infrared and Raman spectra of UO(2)(NH(3))(2)(NO(3))(2) with (14)NH(3)/(15)NH(3) isotopic substitution were measured. The structure was optimized and the vibrational spectrum was calculated by DFT (B3LYP/6-31G(d)) methodology using relativistic effective core potential for U atom. The results for force constant and vibrational frequencies support the experimental assignments and the proposed model, mainly in the far-infrared region, where the metal-ligand bond and lattice vibrations are observed. Based on the theoretical findings and the observed spectra a structure of distorted D(2h) symmetry with the nitrate group acting like bidentate ligands for the UO(2)(NH(3))(2)(NO(3))(2) is proposed.     

Vibrational spectroscopy of synthetic stercorite H(NH4)Na(PO4)·4H2O--a comparison with the natural cave mineral

In order to mimic the chemical reactions in cave systems, the analogue of the mineral stercorite H(NH(4))Na(PO(4))·4H(2)O has been synthesised. X-ray diffraction of the stercorite analogue matches the stercorite reference pattern. A comparison is made with the vibrational spectra of synthetic stercorite analogue and the natural Cave mineral. The mineral in nature is formed by the reaction of bat guano chemicals on calcite substrates. A single Raman band at 920 cm(-1) (Cave) and 922 cm(-1) (synthesised) defines the presence of hydrogen phosphate in the mineral. In the synthetic stercorite analogue, additional bands are observed and are attributed to the dihydrogen and phosphate anions. The vibrational spectra of synthetic stercorite only partly match that of the natural stercorite. It is suggested that natural stercorite is more pure than that of synthesised stercorite. Antisymmetric stretching bands are observed in the infrared spectrum at 1052, 1097, 1135 and 1173 cm(-1). Raman spectroscopy shows the stercorite mineral is based upon the hydrogen phosphate anion and not the phosphate anion. Raman and infrared bands are found and assigned to PO(4)(3-), H(2)O, OH and NH stretching vibrations. Raman spectroscopy shows the synthetic analogue is similar to the natural mineral. A mechanism for the formation of stercorite is provided.     

Observation of the first hydration layer of isolated cations and anions through the FTIR-ATR difference spectra

The attenuated total reflectance-Fourier transform infrared (ATR-FTIR) difference spectra of the dilute aqueous (NH4)2SO4, Na2SO4, MgSO4, ZnSO4, NaClO4, and Mg(ClO4)2 solutions by pure water were obtained at various concentrations. In the difference spectra of aqueous (NH4)2SO4 solutions, a peak at approximately 3039 cm(-1), two shoulders at approximately 3155 and approximately 2894 cm(-1), and a peak at approximately 1445 cm(-1) were ascribed to N-H stretching and bending vibrations, respectively. A small negative peak was resolved at approximately 3660 cm(-1) in the difference spectra of (NH4)2SO4, which is the sole contribution of SO4(2-) either in the O-H stretching or in the O-H bending region. The positive peaks of the difference spectra in the O-H stretching region for Na2SO4, MgSO4, and ZnSO4 systems, which constantly appeared at approximately 3423, approximately 3136, and approximately 3103 cm(-1) respectively, were suggested to be the contribution of the interactions between metal cations (Na+, Mg2+, and Zn2+) and water molecules, especially from the first hydrated layer of the cations. In the region of 800-1200 cm(-1), the normally infrared-prohibited nu1 (SO4(2-)) band was observed as a weak peak at approximately 981 cm(-1) even at very dilute concentrations (0.10 mol dm(-3)) due to the disturbance of the water molecules hydrated with SO4(2-), even though such a feature may increasingly result from associated ions with increasing concentration. The spectra of the water molecules directly influenced by ClO4-, i.e., mostly the first layer of hydrated water, in NaClO4 and Mg(ClO4)2 solutions were obtained by subtracting the corresponding spectra of the same metal sulfate solutions at the same concentrations from the perchlorate solutions. A positive peak at approximately 3583 +/- 6 cm(-1) and a negative peak at approximately 3184 +/- 25 cm(-1) were obtained as the result of the subtraction. The positive peak was attributed to the water molecules weakly hydrogen-bonded with ClO4-, while the negative one to the reduction of water molecules with fully hydrogen-bonded five-molecule tetrahedral nearest neighbor structure on the introduction of ClO4-.     

Infrared spectroscopy and modeling of co-crystalline CO2·C2H2 aerosol particles. I. The formation and decomposition of co-crystalline CO2·C2H2 aerosol particles

Aerosol particles composed of co-crystalline CO(2)·C(2)H(2) were generated in a bath gas cooling cell at cryogenic temperatures and investigated with infrared spectroscopy between 600 and 4000 cm(-1). Similar to results obtained for thin films of the co-crystal [T. E. Gough and T. E. Rowat, J. Chem. Phys. 109, 6809 (1998)], this phase was found to be metastable and decomposed into pure CO(2) and pure C(2)H(2). These decomposed aerosols were characterized through (i) a comparison to experimentally prepared aerosols of mixed CO(2) and C(2)H(2) of known architectures and (ii) the modeling of infrared spectra. A likely architecture after decomposition are C(2)H(2)-CO(2) core-shell particles with a disk-like shape. The co-crystalline CO(2)·C(2)H(2) aerosols prior to decomposition are modeled and analyzed in detail in the subsequent paper (Part II).     

Reactivity induced at 25 K by low-energy electron irradiation of condensed NH3-CH3COOD (1 : 1) mixture

Chemical reactivity is observed following electron irradiation of a binary mixture of ammonia (NH(3)) and acetic acid (CH(3)COOD) at 25 K, without any subsequent thermal activation, as evidenced by vibrational high resolution electron energy loss spectroscopy (HREELS). Analysis of the HREEL spectra and comparison with infrared and Raman data of different molecules are compatible with glycine formation in its zwitterionic form. The onset for electron induced reaction is found to be at about approximately 13 eV. The mechanisms may involve NH radicals interaction with CH(3)COOD molecules. Then glycine formation does not imply any displacement of reactants, so that it involves only NH(3) and CH(3)COOD neighboring molecules.     

Kinetics of metal organic-ammonia adduct decomposition: implications for group-III nitride MOCVD

We have used infrared spectroscopy to investigate the decomposition of the gas-phase (Me)(3)M:NH(3) (M = Al, Ga, In) adducts from room temperature to 573 K, at reactant concentrations in the nominal range used for Al(Ga,In)N metal organic chemical vapor deposition. At 473-523 K TMAl:NH(3) decomposes quantitatively to yield (Me(2))AlNH(2) and CH(4). Comparison of the experimental and theoretical spectra indicates that the majority of the aluminum metal organic product exists in dimer form, i.e., [(Me(2))AlNH(2)](2). The decomposition reaction exhibits unimolecular decomposition kinetics with rate constant parameters of nu = 1 x 10(12) s(-1) and E(a) = 25.7 kcal/mol. At temperatures <543 K, TMGa + NH(3) and TMIn + NH(3) mixtures are dominated by reversible adduct formation-dissociation with no detectable quantities of CH(4) produced. At 574 K a small amount of decomposition is observed in TMGa + NH(3) mixtures, which can be explained by a simple kinetic model that includes the effect of adduct equilibrium. Results demonstrate that the (Me)(3)Al:NH(3) decomposition rate is fast enough to contribute to the early stages of a concerted parasitic chemical reaction mechanism, but the (Me)(3)Ga:NH(3) decomposition rate is too slow.     

Spontaneous self-association of adenine and uracil in polycrystals from low temperature FTIR spectra in the range below 1000 cm(-1)

FTIR spectra of solid samples of co-crystallized adenine and uracil were measured at 10K in the range below 1000cm(-1). New bands ascribable to the N3H (uracil) and NH(2) (adenine) out of plane vibrations, which disappear upon D-exchange, were revealed in comparison with the spectra of pure polycrystalline adenine and uracil obtained in the same conditions. The observed changes relate to the same groups that establish the H-bonds in base pairs of naturally occurring nucleic acids, despite the presence of an extra proton donor NH-group in both molecules. The well-established empirical correlation between the out of plane NH vibrational frequencies and H-bond energies was successfully applied for estimation of the latter in the mixed crystal.     

Single-conformation ultraviolet and infrared spectroscopy of model synthetic foldamers: beta-peptides Ac-beta3-hPhe-beta3-hAla-NHMe and Ac-beta3-hAla-beta3-hPhe-NHMe

The conformational preferences and infrared and ultraviolet spectral signatures of two model beta-peptides, Ac-beta3-hPhe-beta3-hAla-NHMe (1) and Ac-beta3-hAla-beta3-hPhe-NHMe (2), have been explored under jet-cooled, isolated-molecule conditions. The mass-resolved, resonant two-photon ionization spectra of the two molecules were recorded in the region of the S0-S1 origin of the phenyl substituents (37,200-37,800 cm(-1)). UV-UV hole-burning spectroscopy was used to determine the ultraviolet spectral signatures of five conformational isomers of both 1 and 2. Transitions due to two conformers (labeled A and B) dominate the R2PI spectra of each molecule, while the other three are minor conformers (C-E) with transitions a factor of 3-5 smaller. Resonant ion-dip infrared spectroscopy was used to obtain single-conformation infrared spectra in the 3300-3700 cm(-1) region. The infrared spectra showed patterns of NH stretch transitions characteristic of the number and type of intramolecular H-bonds present in the beta-peptide backbone. For comparison with experiment, full optimizations of low-lying minima of both molecules were carried out at DFT B3LYP/6-31+G*, followed by single point MP2/6-31+G* and selected MP2/aug-cc-pVDZ calculations at the DFT optimized geometries. Calculated harmonic vibrational frequencies and infrared intensities for the amide NH stretch vibrations were used to determine the beta-peptide backbone structures for nine of the ten observed conformers. Conformers 1B, 1D, and 2A were assigned to double ring structures containing two C6 H-bonded rings (C6a/C6a), conformers 1A and 2B are C10 single H-bonded rings, conformers 1C and 2D are double ring structures composed of two C8 H-bonded rings (C8/C8), and conformers 1E and 2E are double ring/double acceptor structures in which two NH groups H-bond to the same C=O group, thereby weakening both H-bonds. Both 1E and 2E are tentatively assigned to C6/C8 double ring/double acceptor structures, although C8/C12 structures cannot be ruled out unequivocally. Finally, no firm conformational assignment has been made for conformer 2C whose unusual infrared spectrum contains one very strong H-bond with NH stretch frequency at 3309 cm(-1), a second H-bonded NH stretch fundamental of more typical value (3399 cm(-1)), and a third fundamental at 3440 cm(-1), below that typical of a branched-chain free NH. The single conformation spectra provide characteristic wavenumber ranges for the amide NH stretch fundamentals ascribed to C6 (3378-3415 cm(-1)), C8 (3339-3369 cm(-1)), and C10 (3381-3390 cm(-1)) H-bonded rings.     

Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the M?ller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.     

Hydrogen bonding and the inductive effect in crystalline and solution phases of hexylamine:LiCF3SO3 and Dipropylamine:LiCF3SO3: application to branched poly(ethylenimine)

Raman and infrared spectroscopy were used to study the nature of hydrogen bonding and the cation inductive effect in solutions of LiCF(3)SO(3) dissolved in hexylamine, a primary amine, and dipropylamine, a secondary amine. Comparison of pure hexylamine and hexylamine dissolved in CCl(4) established that the Raman band maximum of the symmetric stretching mode, nu(s)(NH(2)), in pure hexylamine originates in molecules undergoing no hydrogen bonding interactions. The addition of LiCF(3)SO(3) to hexylamine or dipropylamine shifts the frequencies of the solvent NH stretching modes by two effects: the breaking of hydrogen bonds and the cation inductive effect. Comparison of the infrared and Raman spectra allows (to some degree) the separation of these two effects. During these studies, crystalline compounds of hexylamine:LiCF(3)SO(3) and dipropylamine:LiCF(3)SO(3) were discovered, and their structures were solved by single-crystal X-ray diffraction techniques. Vibrational spectra of these crystals and detailed structural knowledge of the cation-solvent interactions complement analogous spectroscopic studies of the solution phases. The cation-polymer and hydrogen bonding interactions of branched poly(ethylenimine) (BPEI) complexed with LiCF(3)SO(3) were modeled by the solutions of hexylamine and dipropylamine containing dissolved LiCF(3)SO(3). Specifically, lithium ion interactions with the primary and secondary amine groups in BPEI were modeled by the solution studies with hexylamine and dipropylamine, respectively. The analysis of the hexylamine system was particularly useful because the primary amine group of BPEI dominates the NH stretching region of the spectrum.     

Infrared absorption of CH3OSO detected with time-resolved Fourier-transform spectroscopy

A step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to detect temporally resolved infrared absorption spectra of CH(3)OSO produced upon irradiation of a flowing gaseous mixture of CH(3)OS(O)Cl in N(2) or CO(2) at 248 nm. Two intense transient features with origins near 1152 and 994 cm(-1) are assigned to syn-CH(3)OSO; the former is attributed to overlapping bands at 1154 ± 3 and 1151 ± 3 cm(-1), assigned to the S=O stretching mixed with CH(3) rocking (ν(8)) and the S=O stretching mixed with CH(3) wagging (ν(9)) modes, respectively, and the latter to the C-O stretching (ν(10)) mode at 994 ± 6 cm(-1). Two weak bands at 2991 ± 6 and 2956 ± 3 cm(-1) are assigned as the CH(3) antisymmetric stretching (ν(2)) and symmetric stretching (ν(3)) modes, respectively. Observed vibrational transition wavenumbers agree satisfactorily with those predicted with quantum-chemical calculations at level B3P86∕aug-cc-pVTZ. Based on rotational parameters predicted at that level, the simulated rotational contours of these bands agree satisfactorily with experimental results. The simulation indicates that the S=O stretching mode of anti-CH(3)OSO near 1164 cm(-1) likely makes a small contribution to the observed band near 1152 cm(-1). A simple kinetic model of self-reaction is employed to account for the decay of CH(3)OSO and yields a second-order rate coefficient k=(4 ± 2)×10(-10) cm(3)molecule(-1)s(-1).     

Etude spectroscopique de derives mercuriques d'amides-thiols: I. Spectres de vibration et structure des thiol-2 N-méthylacétamide,thiol-3 N-méthylpropanamide et N-(thiol-2 éthyle) acétamide

Raman and infrared spectra of CH₃NHCOCH₂SH, CH₃NHCO(CH₂)₂SH and CH₃CONH(CH₂)₂SH have been recorded between 3800 and 200 cm^?1. Some structural information is obtained from their analysis: for pure liquids or solids, molecules form linear chains with NH ? OC hydrogen bonds, the SH group being probably bound to the oxygen of an adjacent molecule. For CCl₄ solutions, an intramolecular hydrogen bond NH ? S is observed for the first compound only, corresponding to the formation of a five-membered ring.     

Crystallization pathways of sulfate-nitrate-ammonium aerosol particles

Crystallization experiments are conducted for aerosol particles composed of aqueous mixtures of (NH(4))(2)SO(4)(aq) and NH(4)NO(3)(aq), (NH(4))(2)SO(4)(aq) and NH(4)HSO(4)(aq), and NH(4)NO(3)(aq) and NH(4)HSO(4)(aq). Depending on the aqueous composition, crystals of (NH(4))(2)SO(4)(s), (NH(4))(3)H(SO(4))(2)(s), NH(4)HSO(4)(s), NH(4)NO(3)(s), 2NH(4)NO(3) x (NH(4))(2)SO(4)(s), and 3NH(4)NO(3) x (NH(4))(2)SO(4)(s) are formed. Although particles of NH(4)NO(3)(aq) and NH(4)HSO(4)(aq) do not crystallize even at 1% relative humidity, additions of 0.05 mol fraction SO(4)(2-)(aq) or NO(3)(-)(aq) ions promote crystallization, respectively. 2NH(4)NO(3) x (NH(4))(2)SO(4)(s) and (NH(4))(3)H(SO(4))(2)(s) appear to serve as good heterogeneous nuclei for NH(4)NO(3)(s) and NH(4)HSO(4)(s), respectively. 2NH(4)NO(3) x (NH(4))(2)SO(4)(s) crystallizes over a greater range of aqueous compositions than 3NH(4)NO(3) x (NH(4))(2)SO(4)(s). An infrared aerosol spectrum is provided for each solid based upon a linear decomposition analysis of the recorded spectra. Small nonzero residuals occur in the analysis because aerosol spectra depend on particle morphology, which changes slightly across the range of compositions studied. In addition, several of the mixed compositions crystallize with residual aqueous water of up to 5% particle mass. We attribute this water content to enclosed water pockets. The results provide further insights into the nonlinear crystallization pathways of sulfate-nitrate-ammonium aerosol particles.     

Hygroscopic growth of multicomponent aerosol particles influenced by several cycles of relative humidity

Infrared aerosol flow tube experiments were performed for mixtures of ammonium, sulfate, and hydrogen ions at 293 K. The impact of the cycling of relative humidity (RH) on the crystals formed and on the hygroscopic growth was evaluated. Submicron particles having an extent of neutralization (X) between 0.60 and 0.75 were the focus, with special emphasis on the composition of aqueous letovicite (NH4)3H(SO4)2 (X = 0.75) because of its unique behavior. Aqueous letovicite particles crystallized initially as an external mixture of solid particles, forming pure particles of letovicite (NH4)3H(SO4)2(s) (LET) in some cases and internally mixed particles of ammonium sulfate ((NH4)2SO4(s);AS) and ammonium bisulfate (NH4HSO4(s); AHS) in other cases. Cycling between 3% and 48% RH increased the fraction of LET particles in the aerosol population, moving in the direction of the more thermodynamically favored species. However, some internally mixed particles remained even after multiple cycles, possibly indicative of a memory effect of AS as a heterogeneous nucleus for AHS. For all compositions studied, the RH of first water uptake and the magnitude of water uptake at higher RH were compared to model predictions. As expected, the more acidic particles (X = 0.60 and 0.65) took up water at the eutonic RH (37%) of mixed AHS/LET particles, but not as expected, both solids dissolved completely, arguing for an increased water solubility possibly attributable to nanocrystalline materials. Particles of X = 0.70 took up water above 41% RH, suggesting a particle morphology of an outer coating of AHS that prevents water uptake at the lower eutonic RH values of mixed AHS/LET and AHS/AS particles. Particles of X = 0.75 took up water as expected for an externally mixed particle population of LET and AS/AHS particles, although the fraction of each type in the population depended on the RH history. These results show that the hysteresis effect for some particles depends on a multi-node RH history. The implication for atmospheric particles is that the crystals present therein as well as particle morphology, water content, and extent of internal/external mixing might continue to evolve during multiple atmospheric cycles of RH.     

Interfacial approach to polyaromatic hydrocarbon toxicity: phosphoglyceride and cholesterol monolayer response to phenantrene, anthracene, pyrene, chrysene, and benzo[a]pyrene

Interactions of phenantrene, anthracene, pyrene, chrysene, and benzo[a]pyrene (polyaromatic hydrocarbons) with model phospholipid membranes were probed using the Langmuir technique. The lipid monolayers were prepared using 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol, 1,2-dipalmitoyl-sn-glycero-3-phosphoserine, 1,2-myristoyl-sn-glycero-3-phosphoethanolamine, 1,2-dilauroyl-sn-glycero-3-phosphocholine, and cholesterol. Surface pressure and electrical surface potential were measured on mixed phospholipid/PAH monolayers spread on a pure water subphase. The morphology of the mixed monolayers was followed with Brewster angle microscopy. Polarization-modulation infrared reflection-absorption spectroscopy spectra obtained on DPPE/benzo[a]pyrene showed that the latter interacts with the carbonyl groups of the phospholipid. On the other hand, the activity of phospholipase A2 toward DLPC used as a probe to locate benzo[a]pyrene in the monolayers indicates that the polyaromatic hydrocarbons are not accessible to the enzyme. The results obtained show that all PAHs studied affect the properties of the pure lipid, albeit in different ways. The most notable effects, namely, film fluidization and morphology changes, were observed with benzo[a]pyrene. In contrast, the complexity of mixed lipid monolayers makes the effect of PAHs difficult to detect. It can be assumed that the differences observed between PAHs in monolayers correlate with their toxicity.     

Crystal and molecular structure of DL-serine hydrochloride studied by X-ray diffraction, low-temperature Fourier transform infrared spectroscopy and DFT(B3LYP) calculations

The structure of dl-serine.HCl was studied by three complementary techniques. Experimental Fourier transform infrared (FT-IR) spectra of pure NH/OH polycrystalline dl-serine.HCl [HO-CH2-CH(NH3+)-COOH.Cl(-)] and the respective deuterated derivatives [ND/ODAlcohol/Acid (<10% and ca. 60% D)] were recorded in the region 4000-400 cm(-1) in the temperature range 300-10 K and interpreted. The assignments were confirmed by comparison with the vibrational spectra of crystalline dl- and l-serine zwitterions [HO-CH 2-CH(NH3+)-COO(-)]. Further insight into the structure of the title compound was provided by theoretical DFT(B3LYP)/6-311++G(d,p) calculations of the infrared spectra and energies of 13 different conformers. Potential energy distributions resulting from normal co-ordinate analysis were calculated for the most stable conformer ( I) in its hydrogenated and deuterated modification. Frequencies of several vibrational modes were used in the estimation of enthalpies of individual H-bonds present in the crystal, using empirical correlations between enthalpy and the frequency shift that occurs as a result of the establishment of the H-bonds. X-ray crystallography data for dl-serine.HCl were recorded for the first time and, together with the experimental vibrational spectra and the theoretical calculations, allowed a detailed characterization of its molecular structure.     

Temperature, composition, and hydrogen isotope effect in the hydrogenation of CO on amorphous ice surface at 10-20 K

An experiment on the addition reaction of a D atom (deuteration) to CO on a cold ice surface is performed by deuterium atom exposure of three types of samples (pure solid CO, CO-capped H2O ice, and CO-H2O mixed ice) at 10-20 K. The variation of IR absorption spectra for the samples was measured by a Fourier transform infrared spectrometer during exposure to deuterium atoms. Reactions on pure solid CO were observed only at 10 K, while reactions on CO-capped H2O ice and CO-H2O mixed ice were observed to proceed even at 20 K. This indicates that the coexistence of H2O at the surface raises the reactive temperature. In addition, the experiment on H atom exposure was also carried out at 15 K to compare the reaction rate constant between the H and D atoms. The ratio of reaction rate constant kD/kH obtained is about 0.08 at 15 K. The authors provide information on the potential energy for the H+CO reaction at the surface by using the ratio kD/kH and by a model calculation of the potential tunneling with the asymmetric Eckart potential.     

Infrared photodissociation spectroscopy of protonated ammonia cluster ions, NH4+(NH3)n (n=5-8), by using infrared free electron laser

Infrared photodissociation action spectra of protonated ammonia cluster ions, NH(4) (+)(NH(3))(n) (n=5-8), were measured in the range of 1020-1210 cm(-1) by using a tunable infrared free electron laser. Analyses by the density functional theory (DFT) show that the spectral features observed can be assigned to the nu(2) vibrational mode of the NH(3) molecules in NH(4) (+)(NH(3))(n). Size dependence of the spectra supports structural models obtained by the DFT calculations, in which the NH(4) (+) ion is solvated by the four nearest-neighbor NH(3) molecules. For NH(4) (+)(NH(3))(5), the spectrum between 1000 and 1700 cm(-1) was measured. The nu(4) bands of the NH(3) molecules and the NH(4) (+) ion were found in the range of 1420-1700 cm(-1).