Dimer Bands Research Articles

Water in krypton matrices

The infrared spectrum of water (H 2O, HDO, D 2O) in krypton matrices has been studied. Bands due to rotating monomers, non-rotating monomers, nitrogen-induced bands, dimer bands and trimer bands are assigned. The results are compared with argon data.

A NEW ATOMIC STRUCTURE MODEL OF Si {001} 2×1 SURFACE

In this papar,a new atomic structure model of the Si{001}2×1 surface determined from band theory calcution of low-energy-electron diffraction are prensented.The new structure model involves a dimer band length ld = 2.387? and thee atomic layer distances d1 = 0.50±0.01?, d2= 0.96±0.01?, d3 = 1.17±0.01? and two asymmetric displacements of dimer atoms.

Optical spectra of Yb atoms and dimers in rare gas matrices

Matrix isolated Yb vapor spectra show absorptions due to atoms, dimers, and higher aggregates. The atomic spectrum reveals strong 395 and 344 nm bands and very sharp weaker 267 and 255 nm absorptions in solid argon. The spectrum of Yb2 shows extensive vibrational stucture around 550 nm both in Ar and Kr matrices with a spacing of 48 cm−1. This absorption is assigned to a 6s2(1S0)6s2(1S0)(1Σ+g) →6s2(1S0)6s6p(1P1)(1Σ+u) transition similar to the one observed for group IIA metal dimers. Higher aggregate spectra overlap with the 550 nm dimer and weak atomic band.

The intermolecular bending vibrations of the hydrogen cyanide dimer

The ν 1+ν 9 1 combination band of the hydrogen cyanide dimer has been observed under sub-Doppler resolution conditions. This mode corresponds to the simultaneous excitation of one quantum in both the “free” CH stretch and the ν 9 1 bending mode of the dimer. When combined with previous work on the ν 1 fundamental and ν 1+ν 9 1-ν 9 1 hot bands of the dimer, the bending frequency can be determined in both the ground (40.7511 cm −1) and excited (40.5520 cm −1) intramolecular vibrational states.

The spectroscopic behaviour of J-aggregates of cyanine dyes without long chains incorporated in the monolayer assemblies

Cyanine dyes without long alkyl chains can be incorporated into composite monolayers with stearic acid at the air-water interface. In the composite monolayer of a 1:1 mixture of dye I (5,5′-dichloro-3,3′,9-triethylthiacarbocyanine chloride) and stearic acid on the water surface, two J-bands were observed, depending on the surface pressure. In the case of dye II (5,5′-dichloro-3,3′-diethyl-9-phehylthiacarbocyanine chloride) the J-aggregate was more stable than those of dye I. The composite monolayers including the dyes can be transferred onto a solid support, retaining the J-aggregate between stearic acid layers. When the temperature was raised above 55°C, the J-band decreased very rapidly with a simultaneous increase in dimer and monomer bands. These spectral changes can be repeated with good reversibility. The correlation between the absorption spectra and the formation of J-aggregates for dyes I and II is discussed.

Spectra and structure of nitrous oxide dimers and clusters

Supersonic expansions of nitrous oxide seeded in argon were studied by single pass FTIR absorption at right angles with the jet axis. In very dilute mixtures, 0.2% N2O, dimer bands at 1282.4 and 2228.0 cm−1 were observed corresponding to the ν1 and ν3 stretches of the monomer. Cluster bands (due to clusters of undetermined size) were readily observed in 20%–100% N2O mixtures at 1165.1, 1295.2, 2245.6, 2467.4, 2578.3, 2811.7, and 3505.8 cm−1. All bands are shifted from the corresponding monomer bands more than in the crystal and in the same direction. Changing the dilution from 10% to 20% shifted all cluster bands 1–2 cm−1 towards low frequencies but still far away from the crystal frequencies. This is interpreted as being due to cluster melting. No increase in bandwidth is associated with the shifts, indicating that, if present at all, the coexistence region described by Berry and co-workers is rather limited in N2O clusters.

The infrared spectrum of matrix isolated guaiacol: self-association and complexation with nitrogen

The self-association of guaiacol and guaiacol- d 3 under cryogenic matrix conditions, and the complexation with the low-temperature nitrogen matrix medium has been studied using IR spectroscopy. In the OH stretching region matrices with high M/A ratios display spectra of almost purely monomeric guaiacol. Lowering the M/A ratio results in self-association, the dominant aggregates being the dimer and the trimer. Annealing the matrix lowers the intensity of the dimer band and increases the intensity of the trimer band. Further annealing increases the relative amount of trimer and larger aggregates. The self-association tendency for guaiacol is found to be much less pronounced than for phenol. Thus the hydrogen bond in guaiacol is sufficiently strong to ensure the trapping of the molecules solely in the intramolecularly bonded form. The complexation of guaiacol with nitrogen under matrix conditions is seen as a splitting of the OH torsional transition into a multiplet, whereas the same transition in a matrix of pure argon gives rise to a single sharp band. The intensity distribution between the different bands in the torsional multiplet shows temperature dependence.

Photochemical reactions of ClF and BrF with hydrogen in solid argon. Infrared spectra of mixed hydracid dimers

Argon diluted samples of H2 and ClF were codeposited at 10 K, and no reaction was observed. Photolysis produced a sharp 2834.5, 2832.5 cm−1 doublet, in agreement with (HF)(HCl) dimer from HF and HCl codeposition experiments. Irradiation at 14–16 K markedly increased the mixed dimer and produced weak monomer and dimer bands of HCl and HF, mixed trimer absorptions, and a sharp band at 3857.9 cm−1 for the Cl--HF radical complex. In similar experiments with HD, two mixed dimers, (HF)(DCl) and (HCl)(DF), were formed; likewise, photolyzing D2 with ClF produced two mixed dimers of the form (DF)(DCl) and (DCl)(DF). In the H2 and BrF system, photolysis produced a sharp absorption at 3814.6 cm−1, in agreement with (HBr)(HF) dimer from HBr and HF codeposition experiments, and a new 3830.5 cm−1 band due to the Br--HF radical complex. In a parallel study with HD, two mixed dimers, (HBr)(DF) and (DBr)(HF), were observed while only one mixed dimer, (DBr)(DF), was formed by BrF and D2. In complementary experiments with H2 and Cl2, (HCl)2 was observed at 2818.1 and 2816.2 cm−1.

Photoelectron spectroscopy of the nitrogen dimer (N2)2 and clusters (N2)n: N2 dimer revealed as the chromophore in photoionization of condensed nitrogen

The He i photoelectron spectra of gas-phase nitrogen dimer and nitrogen clusters have been measured in a pulsed cluster beam. The dimer (N2)2 is characterized by broad bands with vertical ionization energies which are 0.3±0.1 eV lower than for N2 monomer. The bands observed for a mixture of small clusters, estimated to be of average size N̄=10, are identical to the dimer bands except for further shifts of 0.3 eV to lower ionization energies. The clusters bandwidths and band shapes are virtually the same as measured for thin films of condensed N2, indicating that the nitrogen dimer (N2)2 is the ionization chromophore in each case. This offers support for Haberland’s hypothesis that ionization of any Mn cluster produces the ion M+2Mn−2 provided M is a closed-shell atom or molecule. The theory of electronic relaxation polarization of the dielectric medium, which explains the gas-to-solid ionization energy shifts, is modified for the case of finite clusters and to account for dimer ion formation.

FTIR spectra of ammonia clusters in noble gas matrices

FTIR spectra of ammonia have been studied from 200 to 5200 cm−1 over a wide range of concentration and temperature conditions in solid neon, argon, and nitrogen matrices. Dimer bands appear between monomer and higher aggregate absorptions and exhibit intermediate growth behavior on sample annealing and concentration changes. Comparison of spectra in solid argon at 5 and 12 K shows unrelaxed monomer absorptions at 12 K, which almost completely vanish at 5 K without any difference in the dimer spectrum; this indicates that the NH3 submolecules are relaxed in the matrix-isolated dimer. One antisymmetric and two symmetric N–H stretching modes were observed for the dimer, which follow the 14NH3–15NH3 and NH3–ND3 shifts of their monomer analogs. The dimer N–H stretching modes are intensified by fivefold relative to the dimer umbrella bending mode as compared to the same relative monomer band intensities, which is diagnostic of the hydrogen bonding interaction. The matrix dimer spectra show that one N–H bond from one submolecule and two N–H bonds from the other submolecule are involved in nonlinear, intermolecular hydrogen bonding, and that these inequivalent weakly bonded NH3 submolecules form an asymmetric cyclic dimer structure. The matrix dimer spectra further indicate that classical one-hydrogen bond structures cannot be correct. Finally, trimer and higher clusters probably have similar structures, built from the dimer, based on similar infrared spectra.

Protein heterogeneity in rabbit liver ferritin: Two types of molecular dimers

Native pore-gradient polyacrylamide gel electrophoresis of rabbit liver ferritin reveals the usual single band of molecular monomers, but shows two bands at the position of molecular dimers. The proteins in these three bands were purified by excision from preparative slab gels. All three bands (1) contain considerable amounts of iron-rich ferritin when examined by electron microscopy, (2) show complete identity when reacted with anti-rabbit-ferritin antibodies, and (3) have similar amounts of H-type and L-type ferritin subunits with denaturing polyacrylamide gel electrophoresis. These results establish that there are two classes of ferritin molecular dimers. The larger dimer band uniquely also contains a polypeptide with Mr = 170,000. This unusual type of ferritin heterogeneity seems to be due to the presence of a non-ferritin protein associated only with one class of dimers.

Cyanine Dye-Cyclodextrin Systems. Enhanced Dimerization of the Dye

Abstract 3-Cyclodextrin (CD), as well as γ-CD, enhances dimerization of some cyanine dyes, as revealed by the enhancement of dimer band in absorption spectra. The presence and absence of enhancement are governed by geometrical factors, i.e., the length of central methine chain and the size of the chromophore at each end.

Picosecond fluorescence spectroscopy on excimer formation and excitation energy transfer of pyrene in Langmuir-Blodgett monolayer films

Molecular association of pyrene chromophores has been studied with Langmuir-Blodgett (LB) monolayer films consisting of stearic acid and small amounts of 16-( 1-pyreny1)hexadecanoic acid. Picosecond time-resolved fluorescence spectra have shown four fluorescence bands originating from (1 ) a ground-state dimer (band D with vibrational peaks at 381, 401, and 422 nm), (2) a monomer (band M with vibrational peaks at 377, 397, and 421 mm), ( 3 ) an excimer (a structureless band El with a peak at 420 nm), and (4) another type of excimer (a structureless band E2 with a peak at 470 nm). E, was assigned as a fluorescence from a two-center, sandwich-type excimer which corresponds to the well-known pyrene excimer in solution, while E, was assigned as a fluorescence from a one-center type excimer. In the initial time region (0-200 ps), the D band is dominant, and its decay is associated with a rise of the E2 band. The excimer band El forms much faster than band E,. The fluorescence decay curves of the isolated monomer (band M) are interpreted in terms of two-dimensional energy transfer and trapping, leading to the conclusion that the LB monolayer film exhibits an island structure, Le., a nonuniform distribution of guest molecules.

Hydrogen iodide in argon matrixes

The infrared spectra and HI and DI in argon matrices have been studied in the 11-25 K interval. The effects of concentration changes and of impurities have been investigated. Dimer and trimer bands have been assigned, as well as bands due to binary complexes with water, hydrogen bromide, hydrogen chloride, and atomic and molecular iodine.

Rotational analysis and vibrational predissociation in the ν2 band of HCN dimer

The rovibrational infrared spectrum of the bound C–H stretching vibration, ν2, in the HCN dimer has been analyzed. Observed transition frequencies have been combined with previously recorded microwave data to obtain the following molecular parameters (in cm−1): ν2=3241.5696(8), α2=−0.000 110(1), B″=0.058 233 92(1), B′=0.058 344(1), D″J =0.7013(52)×10−7, DJ =0.6636(18)×10−7. The observed full widths at half-maximum intensity of the observed transitions are consistent with excited state lifetimes of 1.7(4)×10−9 s.

Nonphotochemical hole burning of self-aggregated dimers of chlorophyll a in polystyrene

Nonphotochemical hole burning is reported for several burn frequencies throughout the inhomogeneous profiles of chlorophyll a Q/sub y/(0-0) dimer bands in polystyrene films. The holes burned at T = 1.8 K are stable and show no spontaneous hole filling at short times (approx. 1 h). The hole width associated with the lowest dimer component has a value of 0.16 +/- 0.03 cm/sup -1/ at 1.8 K. 17 references, 1 figure.

The self-association of phenol under cryogenic matrix conditions

The self-association of phenol in low-temperature argon and nitrogen matrices has been studied using IR spectroscopy. At high M/A ratios, spectra of almost pure monomeric phenol were obtained; lowering the M/A ratio resulted in increasing self-association. Under the sample preparation conditions used, the dominating aggregate was found to be the dimer, although several submaxima in the OH stretching region show that also multimers are present. Annealing the matrix left the intensity of the dimer band almost unaffected, while the amount of higher aggregates increased. For phenol in nitrogen matrix the OH torsion band shows a fine structure, which persisted in an argon matrix doped with 10% nitrogen. In a matrix of pure argon, however, only a single sharp band was observed for this transition.

Intersubunit disulfides of the follitropin receptor.

The electrophoretic mobility of radioiodinated follitropin (FSH) alpha and beta subunits as well as the alpha beta dimer changed markedly depending on the concentration of reducing agents such as dithiothreitol. The changes were more dramatic in the beta subunit than in the alpha subunit. 125I-FSH, complexed to the receptor on porcine granulosa cells or in Triton X-100 extracts, was cross-linked with a cleavable (nondisulfide) homobifunctional reagent, solubilized in sodium dodecyl sulfate without reducing agents, and electrophoresed. The cross-linked sample revealed three bands of high molecular mass, in addition to the hormone subunit and dimer bands. The band of lightest mass, 110 kDa, was the major band and the other two of 76 and 62 kDa were barely noticeable. Upon reduction with dithiothreitol, the 110-kDa band decreased while the 76- and 62-kDa bands increased, indicating the existence of disulfides between components of the 110-kDa complex. Formation of the disulfide-linked complexes requires 125I-FSH, specifically bound to the hormone receptor and cross-linking, and can be prevented with an excess of native FSH but not human choriogonadotropin. Complex formation was independent of blocking free sulfhydryl groups with N-ethylmaleimide. When the cross-linked complexes were reduced in the gel matrix and analyzed on fresh gels, the 76- and 62-kDa complexes were generated from the 110-kDa band, indicating the loss of two components. The lost components were estimated to be at 14 and 34 kDa. The rate of formation and cleavage of the cross-linked complexes indicated a sequential and incremental addition of 22-, 14-, and 34-kDa components to the FSH alpha beta dimer. The results of reduction of the cross-linked complexes demonstrate the existence of disulfide linkage between the three components.

Composition of cross-linked 125I-follitropin-receptor complexes.

Both of the alpha and beta subunits of intact human follitropin (FSH) were radioiodinated with 125I-sodium iodide and chloramine-T and could be resolved on sodium dodecyl sulfate-polyacrylamide gels. Radioiodinated FSH was affinity-cross-linked with a cleavable (nondisulfide) homobifunctional reagent to its membrane receptor on the porcine granulosa cell surface as well as to a Triton X-100-solubilized form of the receptor. Cross-linked samples revealed three additional bands of slower electrophoretic mobility, corresponding to 65, 83, and 117 kDa, in addition to the hormone bands. The hormone alpha beta dimer band corresponded to 43 kDa. Formation of the three bands requires the 125I-hormone to bind specifically to the receptor with subsequent cross-linking. Binding was prevented by an excess of the native hormone but not by other hormones. A monofunctional analog of the cross-linking reagent failed to produce the three bands. Reagent concentration-dependent cross-linking revealed that their formation was sequential; smaller complexes formed first and then larger ones. When gels of cross-linked complexes were treated to cleave covalent cross-links and then electrophoresed in a second dimension, 18-, 22-, and 34-kDa components were released, in addition to the alpha and beta subunits of the hormone.

Composition and peptide maps of cross-linked human choriogonadotropin-receptor complexes on porcine granulosa cells.

Radioiodinated human choriogonadotropin was affinity-cross-linked with a cleavable (nondisulfide) homobifunctional reagent to the hormone receptor on porcine granulosa cells and the solubilized sample was electrophoresed. Cross-linked samples revealed four additional bands of slower electrophoretic mobility in addition to the hormone alpha, beta, and alpha beta dimer bands. The four bands corresponded to masses of 68, 74, 102, and 136 kDa whereas the alpha beta dimer band corresponded to 50 kDa. Formation of the four bands requires the 125I-hormone to bind specifically to the receptor with subsequent cross-linking. Binding can be prevented by excess of native hormone but not by follitropin. A monofunctional analog of the cross-linking reagent failed to produce the four bands. They were also produced by cross-linking Triton X-100-solubilized hormone-receptor complexes. Reagent concentration-dependent cross-linking revealed that their formation was sequential; smaller complexes formed first and then larger ones. When gels of the cross-linked sample were treated with reagents that cleave covalent cross-links and then electrophoresed in a second dimension gel, 18-, 24-, 28-, and 34-kDa components were released, in addition to the alpha and beta subunits of the native hormone. Simultaneous peptide mapping of the cross-linked complexes in the gel matrix with Staphylococcus V8 protease or papain revealed progressive proteolysis to generate terminal fragments of 30 or 27 kDa, respectively. These fragments were unique to and commonly present in the 74-, 102-, and 136-kDa hormone-receptor complexes but were not produced by proteolysis of the cross-linked human choriogonadotropin (hCG) alpha beta dimer or the hCG alpha subunit. Apparently, the radioactively labeled segment(s) of the alpha subunit of 125I-hCG was cross-linked to the 24-kDa component. The results demonstrate the protein nature of the receptor and suggest that 125I-hCG was initially cross-linked to the 24-kDa component to generate the 74-kDa complex, then the 28- and 34-kDa components were sequentially cross-linked to the 24-kDa component in the 74-kDa complex to generate the 102- and 134-kDa complexes.