Alberto Vivoni

Absorption and Raman spectroscopy of mass‐selected tantalum tetramers in argon matrices

We have examined both the absorption and resonance Raman spectra of mass‐selected Ta tetramers. The tetramers are produced in a sputtering source and mass filtered with a Wien filter, then neutralized and deposited in an Ar matrix at low temperatures. The absorption spectrum indicates two broad transitions, one in the red at 768 nm and another to the blue at 532 nm. Raman resonances could be excited in both regions giving three distinct fundamental frequencies at 270.2(1) cm−1, 185.1(1) cm−1, and 130.6(2) cm−1. The lowest vibration shows a long progression (up to seven overtones) with alternating separations of 135 cm−1 and 126 cm−1, indicative of a weak Jahn–Teller effect in the tetramer ground state. This, along with the observation that the fundamental frequency ratios are close to 2:√2:1 indicate that the molecule has a tetrahedral ground state geometry with an electronic E state symmetry. The appearance of three fundamentals in the resonance Raman spectrum indicates that both excited states correspon...

Normal‐mode analysis of the Raman‐active modes of the anti‐tumor agent 6‐mercaptopurine

The Raman spectra of the antitumor agent 6-mercaptopurine (6MP) in the solid and solution state were recorded in the 600–1700 cm−1 spectral region. The normal modes of 6MP were calculated using a Urey–Bradley empirical force field (UBFF) and with the PM3 semi-empirical method. Prior to performing the normal-mode calculations on 6MP, similar calculations were performed on methanethiol and thiophenol. The force constant parameters obtained from these calculations were then transferred to the 6MP calculations. The results of the normal-mode calculations were compared with the observed spectra and an assignment of the vibrational modes is proposed. Based on changes in the wavenumbers of the Raman spectra between the solution and solid form 6MP and on normal-mode calculations, it is proposed that in the solid form 6MP crystallizes in the tautomer form whereas in solution the molecule ionizes by losing the hydrogen at the N-1 position. Copyright

Determination of the orientation of azathioprine adsorbed on a silver electrode by SERS and ab initio calculations

We report the SERS spectrum of azthioprine (AZA) on a silver electrode surface and the results of normal mode calculations using empirical and ab initio calculations of the 6-mercaptopurione (6-MP) component of AZA. The empirical calculations were done with a Urey-Bradley force field (UBFF) and the ab initio calculations with the STO-3G basis set using the UHF, MP2 and BLYP methods. From the difference between the SERS and solid spectra, we determined that AZA attaches edge-on to the surface through the N3 site on the 6-MP component of the molecule. The UBFF calculation on an Ag adatom-molecule model reproduced most of the main observed frequency shifts in the SERS spectrum. With a similar model, the ab initio calculations yielded frequency shifts in the same direction as the one observed for the in-plane normal modes, but they yielded opposite shifts for the out-of-plane normal modes. This phenomenon may be attributed to a face-on interaction of the 6-MP component with a neighboring adatom made possible by an inclination of the molecule on the surface.

A surface-enhanced Raman study of N-methylquinolinium tricyanoquinodimethanide adsorbed on Ag nanospheres: Determination of molecular orientation and order

Quinolinium tricyanoquinodimethanides are among the most promising molecules for electronic applications. Disorder can be detrimental to the desired electronic properties of a monolayer, and as such, a reliable method to characterize a monolayer without destroying or creating defects is paramount to determining potential applications. Here, the normal and surface-enhanced Raman scattering spectra of N-methylquinolinium tricyanoquinodimethanide (CH₃Q-3CNQ) on silver coated nanosurfaces have been obtained and analyzed. Theoretical treatment of CH₃Q-3CNQ was performed. Optimization and frequency search was conducted using the B3LYP functional with the 6-31G(d) basis set. A complete list of frequencies and assignments for the molecules are presented. The spectroscopic evidence points to the fact that a monolayer of CH₃Q-3CNQ can be formed through the self-assembly process, and the SERS data indicate that the monolayer attaches to the silver surface through the nitrile groups.

Determination of molecular orientation and order of N-(6-Mercaptoacetylhexyl)quinolinium tricyanoquinodimethanide adsorbed on Ag nanoparticles

The surface-enhanced and tip-enhanced Raman scattering spectra of N-(6-Mercaptoacetylhexyl)quinolinium tricyanoquinodimethanides on silver coated nanosurfaces have been obtained, analyzed using Density Functional Theory Calculations, and a complete list of frequencies and assignments for the molecules are presented. The spectroscopic evidence points to the fact that monolayers of the molecule can be formed through the self-assembly process and the SERS data indicate that the monolayer attach to the silver surface through the nitrile groups. SERS spectroscopy was useful in determining the orientation of the monolayer as well as estimating its order. Deprotection the thiol group thereby terminating the tail of the molecule with a sulfur atom allowed for a selectively oriented monolayer to be formed which permanently bound the molecules to the surface preventing rearrangements. This orientation of AcSC6H12Q-3CNQ on silver a surface allowed the electron pairs of the nitrogen to be available for interaction with a second contact. Based on trigonometric tangent function calculations the tilt angle was calculated to be 38° for the protected molecule and 70° for the deprotected alkane thiol monolayer.

Surface-enhanced Raman scattering of a Ag/oligo(phenyleneethynylene)/Ag sandwich.

α,ω-Dithiols are a useful class of compounds in molecular electronics because of their ability to easily adsorb to two metal surfaces, producing a molecular junction. We have prepared Ag nanosphere/oligo(phenyleneethynylene)/Ag sol (AgNS/OPE/Ag sol) and Ag nanowire/oligo(phenyleneethynylene)/Ag sol (AgNW/OPE/Ag sol) sandwiches to simulate the architecture of a molecular electronic device. This was achieved by self-assembly of OPE on the silver nanosurface, deprotection of the terminal sulfur, and deposition of Ag sol atop the monolayer. These sandwiches were then characterized by surface-enhanced Raman scattering (SERS) spectroscopy. The resulting spectra were compared to the bulk spectrum of the dimer and to the Ag nanosurface/OPE SERS spectra. The intensities of the SERS spectra in both systems exhibit a strong dependence on Ag deposition time and the results are also suggestive of intense interparticle coupling of the electromagnetic fields in both the AgNW/OPE/Ag and the AgNS/OPE/Ag systems. Three previously unobserved bands (1219, 1234, 2037 cm(-1)) arose in the SER spectra of the sandwiches and their presence is attributed to the strong enhancement of the electromagnetic field which is predicted from the COSMOL computational package. The 544 cm(-1) disulfide bond which is observed in the spectrum of solid OPE but is absent in the AgNS/OPE/Ag and AgNW/OPE/Ag spectra is indicative of chemisorption of OPE to the nanoparticles through oxidative dissociation of the disulfide bond.

Optimization of force constants with an Urey-Bradley force field avoiding normal mode crossings.

We present a method that simplifies the refinement of force constants in normal mode calculations and makes the results more reliable. The method avoids normal mode crossings by constraining the force constants during refinement. It was tested with pyrrole, imidazole, benzene, pyridine, pyrimidine, aniline and adenine using a Urey-Bradley force field. The global error of the frequency fit for these molecules was 0.61%. The method reproduced with fewer parameters the accuracy of similar calculations of the single ring aromatic compounds. It improved the accuracy and isotopic shifts of previous empirical calculations of adenine by 40%. The C-C and C-N stretchings differed by less than 7% from the values of force constant-bond length empirical relations.