Orientation of Methylguanidinium Ions at the Water-Air Interface

Abstract

We use heterodyne-detected vibrational sum-frequency generation (HD-VSFG) to determine the orientation of the molecular plane of methylguanidinium ions at the surface of aqueous solutions. We measure the VSFG response of the symmetric and antisymmetric methyl stretch vibrations of the methylguanidinium ion with different polarization combinations. We find that for at least 50% of the methylguanidinium ions the molecular plane is at an angle >20° with respect to the surface plane. Hence, for only a minor fraction of the ions does the molecular plane have an orientation (near-)parallel to the surface plane, in contrast to the predictions of recent molecular dynamics simulation studies.

Figure 1

(a) Geometry of our VSFG experiment in a Cartesian lab coordinate system. The incident and emitted beams propagate in the x,z plane. (b) Molecular coordinates (a, b, c) and laboratory coordinates (x, y, z), relevant for the methyl group at water surfaces. The angle θ denotes the tilt angle between the methyl C₃ main symmetry axis (c-axis) and the surface normal z.

Figure 2

Calculated χ_eff⁽²⁾ (β_ccc) as a function of the orientational angle θ for the symmetric methyl vibration assuming that R = 2 (a) and χ_eff⁽²⁾ (β_aca) for the antisymmetric methyl vibration (b). The results are plotted for SSP and PPP polarization combinations. The details of the calculations are given in the text.

Figure 3

Imaginary χ⁽²⁾ spectra of methylguanidine hydrochloride at different polarization combinations: SSP (a) and PPP (b). The global fitting result is shown as the red line, and the contributions of the separate Gaussian peaks are represented by the different colors.

Figure 4

Dependence of D on the center angle of the partial-Gaussian distribution as defined in eq 11 for three different widths σ (a). D as a function of the width (σ) for 45° and 90° center angles (b). A partial-Gaussian angular distributions with θ_c = 90° and a width of ∼150° and δ-function at 45° (c).