Gina M. Florio

Theoretical modeling of the OH stretch infrared spectrum of carboxylic acid dimers based on first-principles anharmonic couplings

Carboxylic acid dimers serve as prototypical systems for modeling the unusual spectral behavior of the hydride stretch fundamental. Large anharmonic effects associated with the pair of cooperatively strengthened OH⋯O=C hydrogen bonds produces complicated infrared spectra in which the OH stretch oscillator strength is spread over hundreds of wave numbers, resulting in a complicated band sub-structure. In this work cubic anharmonic constants are computed along internal coordinates associated with the intramolecular OH stretch, intermolecular stretch, and OH bend internal coordinates for the formic acid and benzoic acid dimers. These are then projected onto the normal coordinates to produce mixed states that are used in computing the OH stretch infrared spectrum. For the benzoic acid dimer the calculations accurately reproduce for three deuterated isotopomers the overall breadth and much of the vibrational sub-structure in the observed spectra. For the formic acid dimer, the spectrum is calculated using a mo...

Scanning Tunneling Microscopy Images of Alkane Derivatives on Graphite: Role of Electronic Effects

Scanning tunneling microscopy (STM) images of self-assembled monolayers of close-packed alkane chains on highly oriented pyrolitic graphite often display an alternating bright and dark spot pattern. Classical simulations suggest that a tilt of the alkane backbone is unstable and, therefore, unlikely to account for the contrast variation. First principles calculations based on density functional theory show that an electronic effect can explain the observed alternation. Furthermore, the asymmetric spot pattern associated with the minimum energy alignment is modulated depending on the registry of the alkane adsorbate relative to the graphite surface, explaining the characteristic moiré pattern that is often observed in STM images with close packed alkyl assemblies.

Hydrogen bonding and tunneling in the 2-pyridone·2-hydroxypyridine dimer. Effect of electronic excitation

Abstract The 2-pyridone·2-hydroxypyridine (2PY·2HP) mixed dimer has been studied using high resolution ultraviolet spectroscopy in the region of the 2PY S1–S0 origin, and fluorescence-dip infrared spectroscopy in the region of the hydride stretch fundamentals. The dense rotational structure of the electronic spectrum is characteristic of a b-type transition with a transition moment at 8°±3° to the b-axis, consistent with excitation of the 2PY half of the dimer. A tunneling splitting of 520±10 MHz appears in the spectrum, due to a double proton transfer in 2PY·2HP. The double proton transfer exchanges the chemical identity of the two monomer units, thereby leading to a double tautomerization. Theoretical calculations suggest that the barrier to such motion is about 8 kcal/mol in the ground state; hence, the observed tautomerization apparently occurs in the excited state. An approximate fit of the high resolution spectrum gives rotational constants that are consistent with an excited state structure in which only the OH⋯O hydrogen bond in the dimer is lengthened substantially. The infrared spectrum out of the pair of ground state zero-point tunneling levels in the XH stretch region is reminiscent of that in the pure (2PY)2 dimer. Its peak absorption frequency is at 2700 cm −1 , but the infrared band is spread over about 500 cm −1 , with reproducible sub-structure due to strong, anharmonic coupling. The excited state spectrum, in contrast, is dominated by a transition at 3135 cm −1 . This band is assigned to the OH fundamental, which is shifted to higher frequency by the weakening of the OH⋯O hydrogen bond upon electronic excitation.

The infrared spectroscopy of hydrogen-bonded bridges: 2-pyridone-(water)n and 2-hydroxypyridine-(water)n clusters, n=1,2

The water-containing clusters of the two tautomers 2-hydroxypyridine (2HP) and 2-pyridone (2PYR) are studied in the hydride stretch region of the infrared using the techniques of resonant ion-dip infrared spectroscopy (RIDIRS) and fluorescence-dip infrared spectroscopy (FDIRS). The results on 2PYR-(water)n build on previous high-resolution ultraviolet spectroscopy [Held and Pratt, J. Am. Chem. Soc. 115, 9708 (1993)] on the n=1,2 clusters and the infrared depletion spectra of Matsuda et al. [J. Chem. Phys. 110, 8397 (1999)] on the n=1 cluster. The 2PYR-W2 FDIR spectrum reflects the consequences of extending and strengthening the H-bonded bridge between N–H and C=O sites in 2PYR. The spectrum shows evidence of strong coupling along the bridge, both in the form of the hydride stretch normal modes and in the breadth of the observed infrared transitions. RIDIR spectra of the 2HP-Wn clusters are compared with those of 2PYR-Wn in order to assess the spectroscopic consequences of forming the analogous water bridg...

Fluorescence-dip IR spectra of jet-cooled benzoic acid dimer in its ground and first excited singlet states.

The IR spectra of three isotopomers of the benzoic acid dimer have been recorded under jet-cooled conditions using the double resonance method of fluorescence-dip IR spectroscopy. In so doing, the spectra are assuredly due exclusively to dimers in the ground-state zero-point level at a rotational temperature of 3-5 K. Even under these conditions, the three isotopomers have remarkably broad spectra, extending from 2600 to almost 3150 cm-1. The spectra show extensive substructure consisting of some 15-20 transitions where only a single OH stretch fundamental should appear in the harmonic limit. The comparison of the undeuterated d0-d0 dimer with the ring-deuterated d5-d5 dimer tests the effect of mixing with the C-H stretches and overtones of the C-H bends. The mixed OH/OD ring-deuterated d6-d5 dimer shifts the frequency and changes the form of the OH stretch normal mode. The analogous OH stretch IR spectrum of the d0-d0 dimer out of the S1 excited-state zero-point level has also been recorded. In this case, much of the closely-spaced substructure is not apparent. What remains is a set of three bands separated from one another by about 180 cm-1. Preliminary results of model calculations of the anharmonic coupling, responsible for the broadening and substructure, are presented. These calculations indicate that it is OH stretch-OH bend coupling, rather than coupling with the intermolecular stretch, that is responsible for much of the observed structure and breadth.

Infrared-induced conformational isomerization and vibrational relaxation dynamics in melatonin and 5-methoxy-N-acetyl tryptophan methyl amide

The conformational isomerization dynamics of melatonin and 5-methoxy N-acetyltryptophan methyl amide (5-methoxy NATMA) have been studied using the methods of IR-UV hole-filling spectroscopy and IR-induced population transfer spectroscopy. Using these techniques, single conformers of melatonin were excited via a well-defined NH stretch fundamental with an IR pump laser. This excess energy was used to drive conformational isomerization. By carrying out the infrared excitation early in a supersonic expansion, the excited molecules were re-cooled into their zero-point levels, partially re-filling the hole created in the ground state population of the excited conformer, and creating gains in population of the other conformers. These changes in population were detected using laser-induced fluorescence downstream in the expansion via an UV probe laser. The isomerization quantum yields for melatonin show some conformation specificity but no hint of vibrational mode specificity. In 5-methoxy NATMA, no isomerization was observed out of the single conformational well populated in the expansion in the absence of the infrared excitation. In order to study the dependence of the isomerization on the cooling rate, the experimental arrangement was modified so that faster cooling conditions could be studied. In this arrangement, the pump and probe lasers were overlapped in space in the high density region of the expansion, and the time dependence of the zero-point level populations of the conformers was probed following selective excitation of a single conformation. The analysis needed to extract isomerization quantum yields from the timing scans was developed and applied to the melatonin timing scans. Comparison between the frequency and time domain isomerization quantum yields under identical experimental conditions produced similar results. Under fast cooling conditions, the product quantum yields were shifted from their values under standard conditions. The results for melatonin are compared with those for N-acetyl tryptophan methyl amide.