PubMed Health⌕ Search

Biomedical subjects

David W Pratt

Publications and source records attributed to David W Pratt.

14 recordsLinked to original sources
PubMed2006

Rotationally resolved electronic spectra of 2- and 3-methylanisole in the gas phase: a study of methyl group internal rotation.

Rotationally resolved fluorescence excitation spectra of several torsional bands in the S1 <-- S0 electronic spectra of 2-methylanisole (2MA) and 3-methylanisole (3MA) have been recorded in the collision-free environment of a molecular beam. Some of the bands can be fit with rigid rotor Hamiltonians; others exhibit perturbations produced by the coupling between the internal rotation of the methyl group and the overall rotation of the entire molecule. Analyses of these data show that 2MA and 3MA both have planar heavy-atom structures; 2MA has trans-disposed methyl and methoxy groups, whereas 3MA has both cis- and trans-disposed substituents. The preferred orientations (staggered or eclipsed) in two of the conformers and the internal rotation barriers of the methyl groups in all three conformers change when they are excited by light. Additionally, the values of the barriers opposing their motion depend on the relative positions of the substituent groups, in both electronic states. In contrast, no torsional motions of the attached methoxy groups were detected. Possible reasons for these behaviors are discussed.

Journal Article↗
PubMed2006

Rotationally resolved S1<-- S0 electronic spectra of fluorene, carbazole, and dibenzofuran: evidence for Herzberg-Teller coupling with the S2 state.

Rotationally resolved fluorescence excitation spectra of the S1 <-- S0 origin bands and higher vibronic bands of fluorene (FLU), carbazole (CAR), and dibenzofuran (DBF) have been observed and assigned. Analyses of these data show that replacement of the CH2 group in FLU with a NH group in CAR and an O atom in DBF produces only localized changes in structure, in the ground state. But the three molecules exhibit different changes in geometry when they are excited by light. The S1 states of the three molecules also are electronically very different. The S1 <-- S0 transition moments of CAR and DBF are parallel to the C2 symmetry axis whereas the corresponding transition moment in FLU is perpendicular to this axis. Herzberg-Teller coupling involving the S2 state also has been observed in the spectra of higher vibronic bands of CAR and DBF. Possible reasons for these behaviors are discussed.

Journal Article↗
PubMed2006

Permanent electric dipole moments of four tryptamine conformers in the gas phase: a new diagnostic of structure and dynamics.

Rotationally resolved electronic spectroscopy in the gas phase, in the absence and presence of an applied electric field, has been used to determine the charge distribution of a cross section of the energy landscape of tryptamine (TRA). We report the magnitude and direction of the permanent electric dipole moments of the four TRA conformers GPyout, GPyup, GPhup, and Antiup in their S0 and S1 electronic states. Each dipole moment is unique, providing a powerful new tool for the conformational analysis of biomolecules in the gas phase. A comparison of the results for the different conformers of TRA reveals that the position and orientation of the ethylamine side chain play a major role in determining both the permanent and induced electric dipole moments of the different species in both electronic states.

Journal Article↗
PubMed2006

On the energy landscapes of 3-indole acetic acid and 3-indole propionic acid. A study of side chain flexibilities in their S(0) and S(1) electronic states.

The tandem of 3-indole acetic acid (IAA) and 3-indole propionic acid (IPA) is ideally suited for a detailed study of the intramolecular forces responsible for the conformational properties of species containing side chains. Toward this end, high resolution S(1)<-- S(0) excitation spectra of the three origin bands in IAA and the two origin bands in IPA were recorded and analyzed. Each origin is assigned to a unique conformer. A discussion of the resulting energy landscape is given.

Indoleacetic Acids↗
PubMed2005

Rotationally resolved electronic spectra of 1,2-dimethoxybenzene and the 1,2-dimethoxybenzene-water complex.

Rotationally resolved S(1) <-- S(0) electronic spectra of 1,2-dimethoxybenzene (DMB) and its water complex have been observed and assigned. The derived values of the rotational constants show that the bare molecule has a planar heavy-atom structure with trans-disposed methoxy groups in its ground and excited electronic states. The transition of DMB is polarized along the b-axis bisecting the methoxy groups, demonstrating that its S(1) state is an (1)L(b) state. Higher energy bands of DMB are also polarized along the b-axis and have been tentatively assigned to different vibrational modes of the (1)L(b) state. The water complex origin appears 127 cm(-1) to the blue of the bare molecule origin. Analyses of the high resolution spectra of DMB/H(2)O and DMB/D(2)O suggest that the water molecule is attached via two O-H...O hydrogen bonds to the methoxy groups in both electronic states. A tunneling motion of the attached water molecule is revealed by a splitting of these spectra into two subbands. Potential barriers to this motion have been determined.

Anisoles↗
PubMed2005

Rotationally resolved electronic spectroscopy of tryptophol in the gas phase.

High resolution S1-S0 fluorescence excitation spectra of tryptophol have been observed in the collision-free environment of a supersonic beam. Each origin band has been assigned to a unique conformer based on its observed rotational constants. Unlike its close relative tryptamine, which exhibits seven distinguishable conformers under similar conditions, tryptophol exhibits only four (GPy-in, GPh-in, and two anti structures). Possible reasons for this difference in behavior are discussed.

Gases↗
PubMed2005

High resolution electronic spectra of 7-azaindole and its Ar atom van der Waals complex.

Rotationally resolved fluorescence excitation spectra of the S(1)<--S(0) origin band of 7-azaindole [1H-pyrrolo(2,3-b)pyridine] and its argon atom van der Waals complex have been recorded and assigned. The derived rotational constants give information about the geometries of the two molecules in both electronic states. The equilibrium position of the argon atom in the azaindole complex is considerably different from its position in the corresponding indole complex. Furthermore, the argon atom moves when the UV photon is absorbed. There are significant differences in the intermolecular potential energy surfaces in the two electronic states. A large, vibration-state-dependent rotation of the S(1)<--S(0) electronic transition moment vector of 7-azaindole relative to that of indole suggests that these differences have their origin in S(1)/S(2) electronic state mixing in the isolated molecule, a mixing that is enhanced by nitrogen substitution in the six-membered ring.

Journal Article↗
PubMed2005

Isomer-specific spectroscopy and conformational isomerization energetics of o-, m-, and p-ethynylstyrenes.

The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and pES) were studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), UV-UV hole-burning spectroscopy (UVHB), resonant ion-dip infrared spectroscopy (RIDIRS), and rotationally resolved fluorescence excitation spectroscopy. In addition, the newly developed method of stimulated emission pumping-population transfer spectroscopy (SEP-PTS) was used to determine the energy threshold to conformational isomerization in m-ethynylstyrene. The S(1) <-- S(0) origin transitions of oES and pES occur at 32 369 and 33 407 cm(-1), respectively. In mES, the cis and trans conformations are calculated to be close in energy. In the R2PI spectrum of mES, the two most prominent peaks (32672 and 32926 cm(-1)) were confirmed by UVHB spectroscopy to be S(1) <-- S(0) origins of these two conformers. The red-shifted conformer was identified as the cis structure by least-squares fitting of the rotationally resolved fluorescence excitation spectrum of the origin band. There are also two possible conformations in oES, but transitions due to only one were observed experimentally, as confirmed by UVHB spectroscopy. Density functional theory calculations (B3LYP/6-31+G) predict that the cis-ortho conformer, in which the substituents point toward each other, is about 8 kJ/mol higher in energy than the trans-ortho isomer, and should only be about 5% of the room temperature population of oES. Ground-state infrared spectra in the C-H stretch region (3000-3300 cm(-1)) of each isomer were obtained with RIDIRS. In all three structural isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance. Infrared spectra were also recorded in the excited electronic state, using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. The first observed and last unobserved transitions in the SEP-PT spectrum were used to place lower and upper bounds on the barrier to cis --> trans isomerization in m-ethynylstyrene of 990-1070 cm(-1). Arguments are given for the lack of a kinetic shift in the measurement. The analogous trans --> cis barrier is in the same range (989-1065 cm(-1)), indicating that the relative energies of the zero-point levels of the two isomers are (E(ZPL)(cis) - E(ZPL)(trans))= -75 to +81 cm(-1). Both the barrier heights and relative energies of the minima are close to those determined by DFT (Becke3LYP/6-31+G) calculations.

Journal Article↗
PubMed2005

Experimental measurement of the induced dipole moment of an isolated molecule in its ground and electronically excited states: indole and indole-H2O.

Reported here are measurements of the magnitude and orientation of the induced dipole moment that is produced when an indole molecule in its ground S(0) and electronically excited S(1) states is polarized by the attachment of a hydrogen bonded water molecule in the gas phase complex indole-H(2)O. For the complex, we find the permanent dipole moment values mu(IW)(S(0)) = 4.4 D and mu(IW)(S(1)) = 4.0 D, values that are substantially different from calculated values based on vector sums of the dipole moments of the component parts. From this result, we derive the induced dipole moment values mu(I) (*)(S(0)) = 0.7 D and mu(I) (*)(S(1)) = 0.5 D. The orientation of the induced moment also is significantly different in the two electronic states. These results are quantitatively reproduced by a purely electrostatic calculation based on ab initio values of multipole moments.

Journal Article↗
PubMed2005

High-resolution electronic spectrum of the p-difluorobenzene-water complex: structure and internal rotation dynamics.

The rotationally resolved S(1) <-- S(0) electronic spectrum of the water complex of p-difluorobenzene (pDFB) has been observed in the collision-free environment of a molecular beam. Analyses of these data show that water forms a planar sigma-bonded complex with pDFB via two points of attachment, a stronger F---H-O hydrogen bond and weaker H---O-H hydrogen bond, involving an ortho hydrogen atom of the ring. Despite the apparent rigidity of this structure, the water molecule also is observed to move within the complex, leading to a splitting of the spectrum into two tunneling subbands. Analyses of these data show that this motion is a combined inversion-internal rotation of the attached water, analogous to the "acceptor-switching" motion in the water dimer. The barriers to this motion are significantly different in the two electronic states owing to changes in the relative strengths of the two hydrogen bonds that hold the complex together.

Journal Article↗
PubMed2004

Charge redistribution on electronic excitation. Dipole moments of cis- and trans-3-aminophenol in their S(0) and S(1) electronic states.

Rotationally resolved electronic spectroscopy in the gas phase, in the absence and presence of an applied electric field, has been used to distinguish the two conformers of 3-aminophenol (3AP) on the basis of differences in their electric dipole moments. cis-3AP has micro = 2.3 D, and trans-3AP has micro = 0.7 D, in their ground electronic states. The two observed values are approximately equal to those expected on the basis of bond dipole additivity rules. However, these rules fail to predict the large change in both the magnitude and the orientation of micro when the two conformers of 3AP absorb light. cis-3AP has micro = 3.3 D, and trans-3AP has micro = 1.7 D, in their excited S(1) electronic states; the angles of orientation of micro with respect to the a inertial axis change by 13 degrees and 38 degrees, respectively. This effect is attributed to (1)L(b)/(1)L(a) state mixing in the S(1) state.

Aminophenols↗
PubMed2004

Proton transfer dynamics via high resolution spectroscopy in the gas phase and instanton calculations.

Tunneling splittings have been observed in the eigenstate-resolved electronic spectrum of the 2-hydroxypyridine/2-pyridone dimer in the gas phase. Deuterium substitution experiments show that these splittings are caused by a concerted double proton transfer reaction along the O-H...O and N...H-N hydrogen bonds that hold the dimer together, substitution of the weaker and longer N...H-N bond having the larger effect. Tunneling splittings calculated by the instanton method for the zero-point level of the ground state are in good agreement with experiment for all observed isotopomers, showing that the dynamics occurs in this state, rather than in the electronically excited state.

Journal Article↗
PubMed2003

Base pair analogs in the gas phase.

A rotationally resolved electronic spectrum of the gas-phase dimer 2-aminopyridine.2-pyridone, an analog of the adenine.thymine base pair, has been observed and assigned, leading to precise measurements of its moments of inertia and preliminary determinations of its structure. A Watson-Crick configuration results, with N...H-N and N-H...O hydrogen bond lengths of 2.898 and 2.810 A, respectively. The two bases are found not to be coplanar; a dihedral angle of 6.1 degrees between the base planes is also estimated from the measured moments of inertia. Possible chemical and biological implications of these results are discussed.

Aminopyridines↗