Dispersed Three-Pulse Infrared Photon Echoes of Nitrous Oxide in Water and Octanol

Abstract

Dispersed IR three-pulse photon echoes due to the antisymmetric (ν3) stretch mode of N2O dissolved in H2O and 1-octanol at room temperature are reported and analyzed. The experimentally determined transition frequency-frequency correlation function (FFCF) in these two solvents is explained in terms of inertial solvent contributions, hydrogen bond network fluctuations, and, for octanol, the motions of the alkyl chains. The H2O hydrogen bond fluctuations result in 1.5 ps FFCF decay, in agreement with relaxation rates determined from photon echo based measurements of other aqueous solutions including salt solutions. In octanol, hydrogen bond fluctuations decay on a slower time scale of 3.3 ps and alkyl chain motions result in an inhomogeneous broadening contribution to the ν3 absorption spectrum that decays on a 35 ps time scale. Rotational reorientation of N2O is nearly 3 times faster in octanol as compared to water. Although the vibrational ν3 N2O absorption line shapes in water and octanol are similar, the line widths result from different coherence loss mechanisms. A hot band contribution in the N2O in octanol solution is found to have a significant effect on the echo spectrum due to its correspondingly stronger transition moment than that of the fundamental transition. The dephasing dynamics of the N2O ν3 stretch mode is of interest as a probe in ultrafast studies of complex or nanoconfined systems with both hydrophobic and hydrophilic regions such as phospholipids, nucleic acids, and proteins. These results demonstrate the value of the N2O molecule to act as a reporter of equilibrium fluctuations in such complex systems particularly due to its solubility characteristics and long vibrational lifetime.

Structure Making and Breaking Effects of Cations in Aqueous Solution: Nitrous Oxide Pump–Probe Measurements

“…Furthermore, N 2 O is a neutral triatomic and is thus a more minimally perturbing probe of aqueous salt solution structure and dynamics than larger molecular species or multiatomic ions, such as N 3 – or SCN – , where ion pair interaction effects may be significant, particularly at high solute concentrations . Finally, from an experimental viewpoint, the N 2 O ν 3 asymmetric stretch 0 → 1 absorption has a large extinction coefficient in aqueous solution (∼1.5 × 10 3 M –1 cm –1 ) and a 18 cm –1 red-shifted excited state frequency in water resulting in strong pump–probe signals at the relatively low N 2 O concentrations (∼50–100 mM) employed for these studies where N 2 O–N 2 O interactions should not be contributing to this solvation environment.…”

Ultrafast Two-Dimensional Infrared Spectroscopy of a Quasifree Rotor: J Scrambling and Perfectly Anticorrelated Cross Peaks

“…S1). [21] The corresponding observed 2DIR spectra of the N 2 O ν 3 mode in SF 6 (ρ* = 0.30) are shown in Fig. 2 for T w = 0.2, 5.0 and 30 ps.…”

Two-Dimensional Infrared Spectroscopy From the Gas to Liquid Phase: Density Dependent J-Scrambling, Vibrational Relaxation, and the Onset of Liquid Character

“…The small absorption feature at ~2209 cm -1 in this spectrum is due a bending n2 (589 cm -1 ) hot band absorption (n2 ® n2 + n3) as noted previously. 61,62 Interestingly, this small hot-band feature is only clearly evident for N2O in liquid phase environments (SF6, H2O and octanol). No rovibrational structure appears in any condensed phase N2O n3 absorption spectrum.…”

“…The rapid fluctuation FFCF dynamics of the N2O n3 mode in liquid SF6 stands in contrast to photon echo results for N2O solvated in octanol or water where significantly slower fluctuation processes are found and instantaneous vibrational frequency memory loss occurs on much longer timescales. 69…”

“…No rovibrational structure appears in any condensed phase N2O n3 absorption spectrum. [61][62][63][64][65] In liquids, the N2O n3 transition frequency is highly sensitive to solvent polarity. 65 The observed peak is at 2221 cm -1 in liquid SF6 and only slightly red-shifted (2220 cm -1 ) from that in the less dense supercritical SF6 at r* = 1.36.…”