S1 Spectra Research Articles

The S(1A g)–S1(1B2u) vibronic transition in benzene: An ab initio study

The four e2g false origin bands and the a1g progression of the S0(1Ag)–S1(1B2u) transition in benzene are simulated ab initio with the recently introduced configuration interaction singles (CIS) with 6-31G orbitals. The ground and excited state CC and CH bond lengths are optimized and compared with the experiment; the CC bond elongation upon excitation is found to be slightly underestimated. The vibrational force fields are calculated at the stationary points of S0 and S1. The 1Ag force field is calculated at the Hartree–Fock level while the 1B2u force field is calculated at the CIS level of theory. The two force fields are scaled to fit the experimental frequencies and the normal mode rotation upon excitation, i.e., the Duschinsky matrix, is obtained. In agreement with previous empirical fitting of the S1(1B2u) vibrational frequencies, the Duschinsky matrix is found to be nearly diagonal with the exception of the b2u modes submatrix which shows a large amount of mixing. The mixing of the b2u modes is larger before scaling but is subsequently reduced after scaling. The normal modes and the optimized geometries are used to calculate the amount of displacement, upon excitation, of the equilibrium position of the totally symmetric modes. This displacement causes the Franck–Condon progression and is slightly underestimated by the calculation. The intensity of the four e2g false origins in the absorption spectrum of S1 is calculated and the Herzberg–Teller intensities of the four bands are found to be very close to the experiment. In particular, the relative intensity of the CCC bend (ν6) and CC stretch (ν8) bands is nicely reproduced. This result is discussed in light of similar calculations at the semiempirical level of theory. We conclude that CIS can be of great value for the unravelling of vibronic spectra of conjugated systems.

Study of sol-gel glass formation and properties by paramagnetic probes

Two types of silica glass doped by o-phenanthroline (o-phen) (S2) and without organic reagent (S1) were studied. Paramagnetic probes (Mn(II), VO(II), Cu(II)) were introduced into both systems by doping or by impregnetion. Sol-gel-xerogel transition in these systems was investigated by electron paramagnetic resonance (EPR). In the S1 system, hydrated metal ions were shown to react weakly with silanol groups during every stage of silica polymerization. The effect of sol to gel7 transition is not observed in S1 spectra. Contrary to that, EPR spectra of VO(II) ions in S2 demonstrate significant changes during sol-gel-xerogel transition. As polymerization proceeds, the signals of non-immobilized hydrated metal ion and that of mixed water-o-phen-metal complex are substituted by signals of metal-o-phen complexes immmobilized on the porous surface. After gelation, the latter signals prevail. EPR spectra provide a means to monitor in situ structural changes during the various stages of glass formation.

Silyl group internal rotation in S1 phenylsilane and phenylsilane cation: Experiments and ab initio calculations

Resonant two-photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure S1–S0 and cation–S1 spectra of internally cold phenylsilane. We measure the adiabatic ionization potentials IP(phenylsilane)=73 680±5 cm−1, IP(phenylsilane ⋅Ar)=73 517±5 cm−1 and IP(phenylsilane ⋅Ar2)=73 359±5 cm−1. We assign many low lying torsion–vibration levels of the S1 (à 1A1) state and of X̃ 2B1 of phenylsilane+. In both states, the pure torsional transitions are well fit by a simple sixfold hindered rotor Hamiltonian. The results for the rotor inertial constant B and internal rotation potential barrier V6 are, in S1, B=2.7±0.2 cm−1 and V6=−44±4 cm−1; in the cation, B=2.7±0.2 cm−1 and V6=+19±3 cm−1. The sign of V6 and the conformation of minimum energy are inferred from spectral intensities of bands terminating on the 3a″1 and 3a″2 torsional levels. In S1 the staggered conformation is most stable, while in the cation ground state the eclipsed conformation is most stable. For all sixfold potentials whose absolute phase is known experimentally, the most stable conformer is staggered in the neutral states (S0 and S1 p-fluorotoluene, S1 toluene, S1 p-fluorotoluene) and eclipsed in the cationic states (ground state toluene+ and phenylsilane+). In phenylsilane+ we estimate several potential energy coupling matrix elements between torsional and vibrational states. For small V6, the term PαPa in the rigid-frame model Hamiltonian strongly mixes the 6a′1 and 6a′2 torsional states, which mediates further torsion–vibrational coupling. In addition, the cation X̃ 2B1 vibrational structure is badly perturbed, apparently by strong vibronic coupling with the low-lying à 2A2 state. Accordingly, ab initio calculations find a substantial in-plane distortion of the equilibrium geometry of the X̃ 2B1 state, while the à 2A2 state is planar and symmetric. The calculations also correctly predict the lowest energy conformer for S0 states and for cation ground states. Finally, we adapt the natural resonance theory (NRT) of Glendening and Weinhold to suggest why sixfold barriers for methyl and silyl rotors are uniformly small, while some threefold barriers are quite large. The phase of the sixfold potential is apparently determined by a subtle competition between two types of rotor-ring potential terms: attractive donor–acceptor interactions and repulsive van der Waals interactions (steric effects).

Picosecond Raman spectroscopy of the B830 LH2 complex ofChromatium purpuratum BN 5500

A setup for generating the Stokes Raman lines of benzene (556, 588 and 624 nm, ∼50 ps) by the use of the second harmonic of a Nd: YLF regenerative amplifier system (527 nm, 70 ps, 1 kHz) has been built. This was then used to detect, for the first time, the picosecond Raman spectrum of a carotenoid bound to an isolated light-harvesting complex of a photosynthetic bacterium. The 527 and 588 nm pulses have been used, respectively, for pumping and probing (delay ∼0 ps) the S1 and T1 states of okenone which is bound to both the isolated B830 LH2 complex and the chromatophores fromChromatium purpuratum BN 5500. Comparison of the above spectra with the S1 and T1 Raman spectra of all-trans-okenone, free inn-hexane solution, shows that only the T1 state is detected with the LH2 complex, and that both the S1 and T1 states are detected with the chromatophores. The results indicate that in the chromatophores there are at least two types of S1 carotenoids with different lifetimes, i.e., one in the LH2 complex which is too short-lived to be detected, most probably due to efficient energy transfer to bacteriochlorophyll, and the other in either the reaction center or the LH1 complex which is long-lived enough to be pumped and probed by 50 ∼ 70 ps pulses. The results also indicate that at least two of the actively light-harvesting carotenoid molecules are in close connection in the isolated LH2 complex since the T1 state is generated through singlet homofission within the short S1 lifetime.

Spectroscopy and nuclear dynamics of tetracene–rare-gas heteroclusters

In this paper we report on the electronic spectroscopy of mass-resolved tetracene⋅Arn (n=1–26) and tetracene⋅Krn (n=1–14) heteroclusters, utilizing two-photon, two-color near-threshold ionization in conjunction with mass-spectrometric detection. The spectra of the S0 → S1 transition and the ionization threshold of these heteroclusters were monitored. The structured spectral features of the S0 → S1 transition of small- and medium-sized (n=1–8) heteroclusters were attributed to the electronic origins of structural isomers and to their intermolecular vibrations. The S0 → S1 spectra of large (n≥9) heteroclusters are broad and were assigned to inhomogeneous broadening due to the coexistence of isomers, with the spectral feature(s) of each distinct isomer being homogeneously broadened. Isomer-specific inhomogeneous line broadening was interrogated by the observation of isomer-specific ionization potentials for medium-sized (n=6–7) heteroclusters and of the dependence of the relative intensities of the spectral features on the conditions of the supersonic expansion. The ionization thresholds of the tetracene⋅An (A=Ar,Kr) reveal a linear (or superlinear) n dependence, being qualitatively different from the sublinear n dependence of the spectral shifts. These different patterns of the size dependence can be traced to the different intermolecular interactions which govern excitation and ionization and to the difference in the charge distribution in S0 and in the positive ion. The experimental spectroscopic data for the spectral shifts and the spectral linewidths were simulated in terms of the first and second moments of the classical line shape, which were obtained from Monte Carlo (MC) constant temperature simulations, in conjunction with a two-parameter fit of the excited-state tetracene–rare-gas potential. The Monte Carlo simulations of the structural fluctuation parameters identified several isomerization phenomena, i.e., correlated restricted and unrestricted surface motion, adcluster isomerization, surface melting and side crossing, and characterized the size dependence of the temperature onsets of these processes for small and medium sized n=2–20 clusters. These isomerization processes could not be interrogated by the investigation of the size dependence of the spectral shifts and linewidths. The size dependence and the isomer specificity of the spectral shifts are well accounted for by the MC simulations. The homogeneous spectral linewidths of small (n<8) clusters pertain to the spectroscopy of ‘‘static’’ isomers, while the line broadening of large (n≥20) clusters manifests inhomogeneous line broadening due to the coexistence of wetting and nonwetting isomers. The temperature dependence of the spectral shifts and inhomogeneous linewidths of large (n≥20) clusters provides means for internal cluster thermometry.

Multiphoton ionization studies of C6H6–(CH3OH)n clusters. I. Comparisons with C6H6–(H2O)n clusters

Resonant two-photon ionization (R2PI) scans of the S0–S1 spectra of C6H6–(CH3OH)n clusters with n=1–5 have been recorded. These scans provide an interesting comparison with earlier spectra from our laboratory on C6H6–(H2O)n clusters. A variety of vibronic level arguments are used to constrain the geometries of the C6H6–(CH3OH)n clusters. The 1:1 and 1:2 clusters possess vibronic level features which are very similar to their aqueous counterparts. The 1:1 cluster places the methanol molecule in a π hydrogen-bonded configuration on or near the sixfold axis of benzene. The spectral characteristics of the 1:2 cluster are consistent with both methanol molecules residing on the same side of the benzene ring as a methanol dimer. Higher C6H6–(CH3OH)n clusters show distinct differences from the corresponding C6H6–(H2O)n clusters. Vibronic level arguments lead to the following conclusions: the methanol molecules in the 1:3 cluster show the strongest hydrogen bonding to the π cloud of any of the clusters and attach to benzene in such a way as to strongly break the sixfold symmetry of its π cloud. The 1:4 clusters are at most only very weakly hydrogen bonded to the π cloud, break benzene’s sixfold symmetry moderately well, and possess strong activity in a very low frequency intermolecular mode. The methanol molecules in the 1:5 cluster show no hydrogen-bonding interaction with benzene’s π cloud, induce remarkably little asymmetry in the π electron density, and produce very little van der Waals’ activity. Monte Carlo simulations using intermolecular potentials developed for liquid simulations serve as a guide to the possible minimum-energy structures for the clusters. The experimental results are used to distinguish between the possible structures. In all cases, the lowest energy structures produced by the calculations satisfactorily fit the vibronic level constraints placed on the structures by our data.

Multiphoton ionization studies of clusters of immiscible liquids. I. C6H6–(H2O)n, n=1,2

Resonant two-photon ionization (R2PI) time-of-flight mass spectroscopy is used to record S0–S1 spectra of the neutral complexes C6H6–H2O, C6H6 –HDO, C6H6–D2O, C6H6–(H2O)2, and C6H6–(D2O)2. In C6H6–H2O, the lack of an S0–S1 origin transition and the presence of a splitting at 610 (which is absent in C6H6 –HDO) provide vibronic level evidence that the water molecule is on the sixfold axis undergoing internal rotation about that axis. Rotational band contour analysis of the 610 transitions of the isotopomers confirms this picture and also determines a ground state center-of-mass separation between C6H6 and D2O of 3.32±0.07 Å, very close to that predicted by ab initio calculations. R2PI scans of the van der Waals structure in the isotopic series C6H6–H2O, C6H6 –HDO, and C6H6–D2O provide tentative assignments for three of the six van der Waals modes in the complex. In C6H6–(H2O)2, rotational band contour analysis of the origin transition provides a best-fit structure in which the two water molecules reside on the same side of the benzene ring at a H2O–H2O separation close to that in the free water dimer. Qualitatively, the structure of the 1:2 cluster is thus one which maximizes the strength of the water–water hydrogen bond at the expense of a somewhat poorer interaction of the second water molecule with the benzene ring in an off-axis geometry. Several intriguing features of the structure are suggested by our analysis, but are near the limit of our ability to distinguish from band contour fitting. Among these features are (i) the on-axis water molecule is pulled slightly in toward the ring from that in the 1:1 complex; (ii) the water dimer prefers an orientation bisecting a C–C bond in the benzene ring; (iii) the water–water separation is ∼0.2 Å less than that in the free water dimer; and (iv) the water dimer axis is tilted by about 10° relative to the plane of the benzene ring. Finally, the van der Waals structure in C6H6–(H2O)2 and C6H6–(D2O)2 suggests the possibility of large amplitude motion in these complexes as well. We postulate that this motion involves a hindered rotation of the on-axis water molecule.

Multiphoton ionization studies of clusters of immiscible liquids. II. C6H6–(H2O)n, n=3–8 and (C6H6)2–(H2O)1,2

Resonant two-photon ionization (R2PI) time-of-flight mass spectroscopy is used to record S0–S1 spectra of the neutral complexes C6H6–(H2O)n with n=3–8 and (C6H6)2–(H2O)1,2. Due to limitations imposed by the size of these clusters, a number of vibronic level arguments are used to constrain the gross features of the geometries of these clusters. Among the spectral clues provided by the data are the frequency shifts of the transitions, their van der Waals structure, the fragmentation of the photoionized clusters, and the complexation-induced origin intensity and 610 splitting. In the 1:3 cluster, simple arguments are made based on the known structures of the 1:1 and 1:2 clusters which lead to the conclusion that all three water molecules reside on the same side of the benzene ring. Three structures for the 1:3 cluster are proposed which are consistent with the available data. Of these, only one is also consistent with the remarkable similarity of the 1:4 and 1:5 spectra to those of the 1:3 cluster. This structure involves a cyclic water trimer in which one of the water molecules is near the sixfold axis in a π hydrogen-bonded configuration. This structure is then expanded in the 1:4 and 1:5 clusters to incorporate the fourth and fifth water molecules in cyclic structures which place the additional water molecules far from the benzene ring without disturbing the interaction of the other water molecules with the benzene ring. For 1:n clusters with n≥6, subtle and then significant changes are observed in the spectra which indicate changes in the way the water cluster interacts with the benzene ring. This development occurs at precisely the water cluster size which calculations predict that cagelike water cluster structures will begin to compete and eventually be favored over large cyclic structures. Finally, cursory scans of the 2:1 cluster show that this cluster also fragments efficiently upon photoionization by loss of a single water molecule and that it possesses a distinctly asymmetric structure with a sizable 610 splitting. Two potential structures are proposed for the 2:1 cluster which can both reasonably explain the observations—a BBW structure in which the water molecule (W) is π hydrogen bonded to a single benzene molecule (B) and a BWB structure in which the water molecule bridges the two benzenes via π hydrogen bonds to both.

Spin-echo proton NMR studies of differential mobility in gizzard myosin and its subfragments

The unexpectedly narrow resonances in the 1H NMR spectra of gizzard myosin, heavy meromyosin, and subfragment 1 were examined by spin-echo NMR spectroscopy. These resonances originated predominantly in the myosin heads, or subfragment 1 units. Smooth muscle myosin undergoes a dramatic change in hydrodynamic properties and can exist either as a folded (10S) or as an extended (6S) species. Factors that influence this transition, namely, ionic strength and phosphorylation (or thiophosphorylation), were varied in the NMR experiments. T2 relaxation experiments on dephosphorylated myosin indicated several components of different relaxation times that were not influenced by changes in ionic strength. Our experiments focused on the components with longer relaxation times, i.e., corresponding to nuclei with more mobility, and these were observed selectively in a spin-echo experiment. With dephosphorylated myosin and HMM, increases in ionic strength caused an increased intensity in several of the narrower resonances. The ionic strength dependence of these changes paralleled that for the 10S to 6S transition. With thiophosphorylated myosin and HMM, changes in ionic strength also influenced the intensities of the narrower resonances, and in addition changes in the 1H NMR spectrum due to thiophosphorylation were observed. The narrow resonances seen with myosin and HMM were observed with S1, but the spin-echo spectra of S1 were not influenced either by changes in ionic strength or by phosphorylation. These results suggest that a fraction of the 1H resonances in smooth muscle myosin and its fragments originates from both aliphatic and aromatic residues of increased mobility compared to the mobility expected from hydrodynamic properties of these proteins. In general, the intensities of these residues increase with increasing ionic strength, and this is consistent with an increase in the percentage of mobile residues during the 10S to 6S transition. Segmental flexibility appeared also to be influenced by phosphorylation within the 6S conformation. These changes were not detected in the isolated myosin heads and thus required a higher order of structure, either the subfragment 2 region or the interaction of myosin heads.

Theoretical studies of van der Waals molecules: the D2-D2 dimer

The M80 potential energy surface of Meyer, Schaefer and Liu (1983) is used to obtain the bound-state energies of dimers of deuterium in both its ortho and para modifications. The quantum mechanical close-coupling equations have been solved by direct numerical integration. Scattering calculations have yielded the energies and widths of rotationally predissociating states of ortho-D2-ortho-D2. That some of these resonances have dual Feshbach/shape character is noted. The dimer structure accompanying the observed near-infrared S1(0) and Q1(0)+S0(0) spectra in ortho-deuterium, is modelled by treating the two D2 molecules as distinguishable rigid rotors. The conclusions, that the 1974 measurements of McKellar and Welsh provide evidence both for rotational splitting and internal rotational predissociation, are examined in the light of new spectroscopic results.

ChemInform Abstract: The S1‐S0(1B2u‐1Ag) Transition of p‐Difluorobenzene Cooled in a Supersonic Free Jet Expansion. Excitation and Dispersed Fluorescence Spectra, Vibrational Assignments, Fermi Resonances, and Forbidden Transitions.

The vibronic spectroscopy of the S1(1B2u)–S0(1Ag) transition of p‐difluorobenzene (000 at 36 838 cm−1) cooled in a supersonic free jet expansion in argon has been reinvestigated in some detail. Analysis of over 50 vibronic transitions using fluorescence excitation and dispersed single vibronic level fluorescence spectroscopy has led to the establishment or confirmation of the assignments of 19 S1 and S0 frequencies, including eight previously unassigned S1 vibrational frequencies, and the reassignment of two S1 and one S0 frequencies. Several Franck–Condon forbidden transitions have been identified. Their activity in the S1–S0 spectrum is attributed to vibronic coupling involving higher lying electronic states. Forbidden transitions involving b3g modes, notably ν27 and ν26, derive their intensity from a higher lying 1B1u electronic state, via vibronic coupling that is analogous to that responsible for the 1B2u–1Ag transition in benzene. Numerous Fermi resonances in both the S1 and S0 states have been iden...

ChemInform Abstract: Intensity Distribution in Rotational Line Spectra. Part 2. Analysis of the Fluctuations in the Doppler‐Free S1 ← S0 Spectrum of Benzene.

ChemInformVolume 20, Issue 7 Physical Organic Chemistry ChemInform Abstract: Intensity Distribution in Rotational Line Spectra. Part 2. Analysis of the Fluctuations in the Doppler-Free S1 ← S0 Spectrum of Benzene. Y. M. ENGEL, Y. M. ENGEL Fritz Haber Res. Cent. Mol. Dyn., Hebrew Univ., Jerusalem 91904, Isr.Search for more papers by this authorR. D. LEVINE, R. D. LEVINE Fritz Haber Res. Cent. Mol. Dyn., Hebrew Univ., Jerusalem 91904, Isr.Search for more papers by this author Y. M. ENGEL, Y. M. ENGEL Fritz Haber Res. Cent. Mol. Dyn., Hebrew Univ., Jerusalem 91904, Isr.Search for more papers by this authorR. D. LEVINE, R. D. LEVINE Fritz Haber Res. Cent. Mol. Dyn., Hebrew Univ., Jerusalem 91904, Isr.Search for more papers by this author First published: February 14, 1989 https://doi.org/10.1002/chin.198907049AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume20, Issue7February 14, 1989 RelatedInformation

The S1–S(1B2u–1A g) transition of p-difluorobenzene cooled in a supersonic free jet expansion. Excitation and dispersed fluorescence spectra, vibrational assignments, Fermi resonances, and forbidden transitions

The vibronic spectroscopy of the S1(1B2u)–S0(1Ag) transition of p-difluorobenzene (000 at 36 838 cm−1) cooled in a supersonic free jet expansion in argon has been reinvestigated in some detail. Analysis of over 50 vibronic transitions using fluorescence excitation and dispersed single vibronic level fluorescence spectroscopy has led to the establishment or confirmation of the assignments of 19 S1 and S0 frequencies, including eight previously unassigned S1 vibrational frequencies, and the reassignment of two S1 and one S0 frequencies. Several Franck–Condon forbidden transitions have been identified. Their activity in the S1–S0 spectrum is attributed to vibronic coupling involving higher lying electronic states. Forbidden transitions involving b3g modes, notably ν27 and ν26, derive their intensity from a higher lying 1B1u electronic state, via vibronic coupling that is analogous to that responsible for the 1B2u–1Ag transition in benzene. Numerous Fermi resonances in both the S1 and S0 states have been identified. The prevalent Fermi resonance between ν′5 and 2ν6 has been analyzed with the assistance of both excitation and dispersed fluorescence spectroscopy, yielding a coupling matrix element [g566<51‖Q5‖50><60‖Q6‖62>]=−1 cm−1. Thirty-one matrix elements describing cubic anharmonicity and involving a variety of vibrational modes have been estimated. The majority of the coupling matrix elements lie within the range ±2 cm−1.

Characterization of the manganese O2-evolving complex and the iron-quinone acceptor complex in photosystem II from a thermophilic cyanobacterium by electron paramagnetic resonance and X-ray absorption spectroscopy.

The Mn donor complex in the S1 and S2 states and the iron-quinone acceptor complex (Fe2+-Q) in O2-evolving photosystem II (PS II) preparations from a thermophilic cyanobacterium, Synechococcus sp., have been studied with X-ray absorption spectroscopy and electron paramagnetic resonance (EPR). Illumination of these preparations at 220-240 K results in formation of a multiline EPR signal very similar to that assigned to a Mn S2 species observed in spinach PS II, together with g = 1.8 and 1.9 EPR signals similar to the Fe2+-QA- acceptor signals seen in spinach PS II. Illumination at 110-160 K does not produce the g = 1.8 or 1.9 EPR signals, nor the multiline or g = 4.1 EPR signals associated with the S2 state of PS II in spinach; however, a signal which peaks at g = 1.6 appears. The most probable assignment of this signal is an altered configuration of the Fe2+-QA- complex. In addition, no donor signal was seen upon warming the 140 K illuminated sample to 215 K. Following continuous illumination at temperatures between 140 and 215 K, the average X-ray absorption Mn K-edge inflection energy changes from 6550 eV for a dark-adapted (S1) sample to 6551 eV for the illuminated (S2) sample. The shift in edge inflection energy indicates an oxidation of Mn, and the absolute edge inflection energies indicate an average Mn oxidation state higher than Mn(II). Upon illumination a significant change was observed in the shape of the features associated with 1s to 3d transitions. The S1 spectrum resembles those of Mn(III) complexes, and the S2 spectrum resembles those of Mn(IV) complexes. The extended X-ray absorption fine structure (EXAFS) spectrum of the Mn complex is similar in the S1 and S2 states. Simulations indicate O or N ligands at 1.75 +/- 0.05 A, transition metal neighbor(s) at 2.73 +/- 0.05 A, which are assumed to be Mn, and terminal ligands which are probably N and O at a range of distances around 2.2 A. The Mn-O bond length of 1.75 A and the transition metal at 2.7 A indicate the presence of a di-mu-oxo-bridged Mn structure. Simulations indicate that a symmetric tetranuclear cluster is unlikely to be present, while binuclear, trinuclear, or highly distorted tetranuclear structures are possible. The striking similarity of these results to those from spinach PS II suggests that the structure of the Mn complex is largely conserved across evolutionarily diverse O2-evolving photosynthetic species.

Spectroscopic characterization of the acceptor state Q −A and the donor state S2 of Photosystem II of spinach in the blue, red and near-infrared

With Photosystem II particles prepared from spinach, partial difference spectra were measured of the primary bound plastoquinone Q A and of the donor state S2. Extinction coefficients were estimated by comparison with the absorption change of a redox-active dye, N, N, N′, N′-tetramethyl- p-phenylenediamine. The spectrum of Q − A − Q A was better resolved than in earlier studies (Van Gorkom, H.J. (1974) Biochim. Biophys. Acta 347, 439–442; Schatz, G.H. and Van Gorkom, H.J. (1985) Biochim. Biophys. Acta 810, 283–294; Dekker, J.P. (1985) Thesis, University of Leiden) and is seen to be largely composed of shifts of two pigment bands in opposite directions, presumably a pheophytin red shift and a chlorophyll blue shift, as in bacterial reaction centers (Clayton, R.K. and Straley, S.C. (1972) Biophys. J. 12, 1221–1234). The spectrum of S2–S1 in the red suggests blue shifts of two chlorophyll bands. It resembles, but is not identical to, the S2–S1 spectrum earlier reported for Photosystem II of Synechococcus (Saygin, O. and Witt, H.T. (1985) FEBS Lett. 187, 224–226). Earlier measurements of S2-associated absorbance in the near-infrared (Dismukes, G.C. and Mathis, P. (1984) FEBS Lett. 178, 51–54) are not confirmed. The observed changes in this range are probably largely scattering changes. This conclusion is reached on the basis of theoretical considerations

On the S0(0) and S1(0) spectra of the H2-H2dimer

The authors present computed frequencies of the S0(0) transitions between bound and pseudo-bound levels of the para-H2-para-H2 dimer. The calculations account for the interchange symmetry of the H2 molecules. Analogous calculations of the S1(0) spectrum and found to be in satisfactory agreement with the laboratory measurements of McKellar and Welsh (1974). The authors' results lend support to the proposed identification of features in the far infrared spectra of Jupiter and Saturn with S0(0) transitions of the H2-H2 dimer.

The structure of electronic excited states in t r a n s-stilbene: Picosecond transient Stokes and anti-Stokes Raman spectra

We measured picosecond transient Stokes and anti-Stokes Raman spectra of S1 trans-stilbene in hexane. We observed 18 vibrational modes in the Stokes spectra and two modes in the low wave number anti-Stokes spectra. The band positions and band shapes of the transient Raman spectra suggest a distribution of molecular conformers in S1 having different single and double bond strengths. The band intensities suggest a redistribution of electron density in Sn that is concentrated in the olefinic region and in the ortho and meta region of the phenyl rings.

Spectral analysis of heart sounds: Relationships between some physical characteristics and frequency spectra of first and second heart sounds in normals and hypertensives

Frequency analysis of heart sounds has been gaining recognition as a possible indicator of several heart and valve diseases, although a comprehensive study of normal heart sounds has not been published. Relating the frequency content of normal heart sounds to certain physical characteristics surrounding the generation of these sounds could lead to a valuable diagnostic tool and give a better understanding of the mechanism of heart sounds production. In this study, the first and second heart sounds from seventy-four normal, and seven hypertensive volunteers were recorded, digitized and analysed using a Fast Fourier Transform algorithm. Statistical analysis was used to relate physical characteristicss (sex, blood pressure, and body surface area) of the subjects to the frequency content of normal heart sounds and to compare normal and hypertensive heart sounds. Statistical analysis showed that the major concentration of energy, for both first heart sound (S1) and second heart sound (S2), is below 150 Hertz (Hz) which may indicate that both sounds are caused by vibrations within the same structure, possibly the entire heart. However S2 spectra have greater amplitude than S1 spectra above 150 Hz, which may be due to vibrations within the aorta and pulmonary artery. Relationships observed between body surface area, sex, blood pressure, and the frequency content of heart sounds indicate that as heart size increases, the amplitude of the frequency coefficients above 150 Hz decreases. These observations were more identifiable in the S1 spectra than in the S2 spectra, possibly because the S2 higher frequency components may mask subtle changes in the S2 spectra caused by heart size changes. However, when the changes in heart size are significant, as in hypertension or increased body surface area, trends in the S2 spectra can be observed.

Theoretical studies of van der Waals molecules: the H2-H2dimer

The M80 potential energy surface of Meyer, Schaefer and Liu (1983) is used to obtain the bound-state energies of dimers of deuterium in both its ortho and para modifications. The quantum mechanical close-coupling equations have been solved by direct numerical integration. Scattering calculations have yielded the energies and widths of rotationally predissociating states of ortho-D2-ortho-D2. That some of these resonances have dual Feshbach/shape character is noted. The dimer structure accompanying the observed near-infrared S1(0) and Q1(0)+S0(0) spectra in ortho-deuterium, is modelled by treating the two D2 molecules as distinguishable rigid rotors. The conclusions, that the 1974 measurements of McKellar and Welsh provide evidence both for rotational splitting and internal rotational predissociation, are examined in the light of new spectroscopic results.

Excitation-Wavelength-Dependent Structure in the Fluorescence Spectra of Bu States of Diphenylpolyenes

Experiments have used a tunable picosecond laser to study evolving fluorescence spectra of 1,8-diphenylocta-l,3,5,7-tetraene (DPO) in hexane solution at room temperture. The spectra recorded synchronously (±20 ps) with the irradiation event are analysed in terms of the time-integrated spectra, spectra obtained through a delayed gate, resonance Raman signals and fluorescence from the second excited singlet state. Data are presented to show the dependence of the S2 → S0 spectrum on the excitation wavelength. The spectra clearly show that vibrational relaxation is incomplete on the timescale of the S2 lifetime. On the other hand, the spectral bands show little tendency at room temperature towards line-narrowing effects. This indicates that the rate of molecular site relaxation is at least comparable to the lifetime of the S2 state under these conditions.