Deriving the complete molecular conformation of self-assembled alkanethiol molecules from sum-frequency generation vibrational spectra

Abstract

The sum frequency generation ͑SFG͒ spectrum of CH 2 and CH 3 modes in the self-assembled monolayer of octadecanethiol is modeled in order to understand the reasons of the very low SFG intensity of CH 2 modes and to investigate quantitatively the two types of L-shaped conformations qualitatively suggested in the literature. CH 2 modes are assumed local in order to calculate easily the spectrum of any conformation, and ab initio molecular hyperpolarizabilities are used. It is found that the absence of CH 2 bands does not imply an all-trans conformation and vice versa. Several conformations are compatible with SFG, but only one of them can be arranged on Au͑111͒ and completely agrees with x-ray diffraction. All conformational details of the L-shape molecules are obtained.

Orientational Analysis of Dodecanethiol and p‐Nitrothiophenol SAMs on Metals with Polarisation‐Dependent SFG Spectroscopy

“…The orientational conformations of the methyl group of DDT and of p-NTP SAMs obtained through such polarisation measurements result to be in very close agreement with those obtained by investigating the intensity ratio of two modes in a single polarisation configuration or by employing other analytical techniques. For example, the analysis of the orientation of methyl-terminated SAMs on Au through the ratio of the CH 3 symmetric and asymmetric SFG signals towards the tilt angle, as calculated by Bourguignon et al, [15] indicates that the ppp spectral profile measured here (Figure 2, top-right) corresponds to a tilt angle of about 478 (468 through the current polarisation-dependent approach); in SAMs on Pt, the same ratio has been calculated by Nogouchi et al, [17] and indicates that the ppp spectral profile measured in Figure 2 (top-left) corresponds to a tilt angle of about 358 (378 through the current polarisation-dependent approach). Moreover, polarisation-dependent SFG measurements have indicated that p-NTP SAMs are tilted towards the metal surfaces (< # > = 608), in agreement with the results obtained on aromatic thiols through other analytical techniques.…”

“…. [15,22,[51][52][53] The vibrations of the methylene CH bonds, expected at 2920 cm À1 and at 2850 cm…”

“…[7,12,15,16,18,19] In this work, one shows that combining ppp with ssp is the only way to deduce both the tilt and rotation angles from the measurements.…”

“…[7,8,15,37,38] Wang et al have clearly discussed the complexity in modelling the Fermi resonance to get its hyperpolarisability tensor elements. Additionally, they have shown that retrieving the molecular orientation by taking into account the Fermi resonance provides orientational data consistent with those obtained by the CH 3 stretching ratios.…”

“…Hence, the systematic application of SFG to metal interfaces has still to be further investigated. [13,14] Until now, the molecular conformations at such interfaces have been inferred from the relative SFG intensity of different vibrational modes, [7,12,[15][16][17][18][19] rather than from the direct measurement of the molecular nonlinear susceptibility components; the latter is the procedure commonly applied for insulating interfaces, while it has never been used for metals. Indeed, some papers have shown SFG spectra collected on metal interfaces with different polarisation sets [12,[20][21][22][23][24] but none of them have retrieved orientation information from such polarisation-dependent SFG data.…”