Duohai Pan

Potential Use of Raman Spectroscopy for Determination of Amylose Content in Maize Starch

ABSTRACT An analytical method using Raman spectroscopy was developed for the determination of amylose concentration in maize starches. FT-Raman spectra of four maize starches with amylose content varying from 3.3 to 66% were obtained. A Raman band at ≈1657 cm-1 correlated linearly with amylose concentration in the four maize starches, and a calibration curve for Raman band intensity versus amylose content was developed. The linear correlation of the I1657/I900 integrated areas with amylose content was r = 0.997. The Raman-based calibration curve allows fast and nondestructive determination of the amylose content in maize starches with minimal sample preparation.

Time-resolved resonance Raman and density functional study of the radical cations of chlorobiphenyls

Abstract We have obtained time-resolved resonance Raman spectra of the radical cations of 4-chlorobiphenyl, 3-chlorobiphenyl, and 2-chlorobiphenyl. We have performed density functional theory B3LYP/6-31G∗∗ calculations to find the optimized structures and predicted vibrational spectra for the ground states of the chlorobiphenyl radical cations. The computational and experimental results suggest that all of these chlorobiphenyl radical cations are nonplanar. The 2-chlorobiphenyl radical cation is noticeably more nonplanar than the other two radical cations and this appears to be due mainly to steric interactions. The radical cations are more noticeably nonplanar than the corresponding T1 states of the neutral molecules.

Investigation of the effects of substitution position on the S0 and T1 states of chlorobiphenyl

Abstract We have performed ab initio molecular orbital calculations to find the optimized geometries and predicted vibrational spectra for the S 0 and T 1 states of 4-chlorobiphenyl, 3-chlorobiphenyl, and 2-chlorobiphenyl. We have obtained Raman spectra of the S 0 and T 1 states of 4-chlorobiphenyl, 3-chlorobiphenyl, and 2-chlorobiphenyl. We find that 2-chlorobiphenyl has a nonplanar structure in its T 1 state while the T 1 states of 4-chlorobiphenyl and 3-chlorobiphenyl have a nearly planar quinoidal structure. Comparison to previous EPR results for methylbiphenyls suggests that the twisting of T 1 2-chlorobiphenyl is likely due to an electron withdrawing effect of the ortho substituted halogen atom.

NANOSECOND TIME-RESOLVED RESONANCE RAMAN INVESTIGATION OF THE T1 TN TRANSITION OF 2-BROMONAPHTHALENE

Abstract We have obtained nanosecond time-resolved resonance Raman spectra of the T 1 →T n transition of 2-bromonaphthalene in methanol solvent. The transient resonance Raman spectra indicate that there are mostly structural changes in the C–C stretch ν 5a vibrational coordinate along with a smaller amount of change in the C–Br stretch and C–Br in plane deformation coordinates. This suggests that the T 1 →T n transition of 2-bromonaphthalene does not initially excite a directly dissociative triplet state and a predissociative mechanism is probably responsible for the efficient T n state C–Br bond cleavage found in previous photochemistry studies.

Investigation of the effects of substitution position on the radical anions of chlorobiphenyls

Abstract We report density functional theory calculations for the radical anions of 4-chlorobiphenyl, 3-chlorobiphenyl, 2-chlorobiphenyl, 2,2′-dichlorobiphenyl, 2-fluorobiphenyl, 2-methylbiphenyl and biphenyl in order to investigate how their structures are affected by substitution position and type of substituent at the ortho position. We also compare the optimized structures for these radical anions to corresponding results for the radical cations and neutral molecules. The chlorobiphenyl radical anions have weaker C–Cl bonds than the radical cations or the T 1 triplet state of the neutral molecule.

Nanosecond time-resolved resonance Raman investigation of the radical cation of 4,4′-dibromobiphenyl and the T1→Tn transition of 4,4′-dibromobiphenyl

Abstract We have taken nanosecond time-resolved resonance Raman spectra of the T 1 →T n transition of 4,4′-dibromobiphenyl in cyclohexane solution and of the radical cation of 4,4′-dibromobiphenyl in acetonitrile solution. The resonance Raman spectrum of T 1 4,4′-dibromobiphenyl is consistent with a planar quinoidal structure. The C–Br bond appears stronger in the T 1 state than in the S 0 ground state and this is in agreement with photochemistry studies done for 4-bromobiphenyl that indicate that the T 1 state must be further excited by excimer formation and/or electron transfer with an appropriate donor prior to a debromination reaction.

FT-surface-enhanced Raman scattering of 2,2′-cyanine adsorbed on silver with AFM characterization of silver films

Abstract Silver films were prepared using the homogeneous chemical reduction method. The surface morphology and optical properties of these films Ag films fabricated with different deposition times were characterized using atomic force microscopy (AFM) and absorption spectroscopy. The AFM images show that the Ag films posses nanometer scale structure and differ mainly in the degree of aggregation of Ag particles. The Ag films exhibit a surface plasmon resonance band at about 380 nm with a red-shifted band that can be extended to 1000 nm with increasing degree of aggregation of Ag particles. FT-SERS spectra were obtained for 2,2′-cyanine adsorbed on the Ag films.

Substituent effects on radical cations of halogenated biphenyl compounds: a density functional theory and time-resolved resonance Raman study

We report density functional theory calculations to obtain the optimized structures for the ground state neutral molecule and radical cations for twelve substituted biphenyl compounds. We have also obtained experimental Raman spectra of the ground state neutral molecule and radical cations for several substituted biphenyl compounds. We investigate how the type of halogen atom (fluoro, chloro, or bromo) and its substitution position influences the structure and properties of the radical cations of halogenated biphenyl compounds. All of the halogenated biphenyl compounds have non-planar radical cations with the degree of planarity increasing for ortho substituted halobiphenyls compared to meta and para substitution due mainly to steric interactions. Multiple ortho substitution increases the degree of non-planarity of the radical cation.

Nanosecond time-resolved resonance Raman spectroscopic and ab initio MO investigation of substituent effects on the triplet states of bromonaphthalenes

Nanosecond time-resolved resonance Raman spectra of the T1 states and Raman spectra of the S0 states of 1-bromonaphthalene and 1,4-dibromonaphthalene were measured. Ab initio calculations were also performed to determine the optimized geometries and vibrational spectra for the S0 and T1 states of 1-bromonaphthalene and 1,4-dibromonaphthalene were predicted. The results were compared with those previously found for 2-bromonaphthalene and it was found that the Br atom substitution position and number both affect the changes in the C— Br bond in the T1 and Tn states. The intensities of the time-resolved resonance Raman spectra indicate that the C— Br bond changes its structure much more in the Tn state relative to the T1 state of 2-bromonaphthalene than 1-bromonaphthalene or 1,4-dibromonaphthalene. Possible implications of these substitution position effects on the ‘reluctant’ bond cleavage processes occurring from the Tn states of bromonaphthalenes are discussed. Copyright