The effect of an
external electric field on the C–O stretch
frequency, ν(C–O), of carbon monoxide was studied for
CO in different environments of condensed molecular films: (I) chemisorbed
CO on Pt(111) covered with amorphous solid water (ASW), (II) CO trapped
in an ASW matrix, (III) chemisorbed CO on Pt(111) covered with solid
Ar, and (IV) CO trapped in a solid Ar matrix. Changes in ν(C–O)
of these samples under an electric field were measured to investigate
the Stark frequency shift and the effect of metal–adsorbate
charge transfer on the frequency change. The electric field was applied
up to 4.3 × 108 V·m–1 using
the ice film capacitor method. Reflection absorption infrared spectroscopy
was used to monitor the spectral changes of the ν(C–O)
band. The Stark shift was measured from the ν(C–O) change
of isolated CO in the ASW matrix under the field. The effect of metal–adsorbate
charge transfer was estimated for chemisorbed CO by measuring the
ν(C–O) shift under the field and subtracting the electrostatic
Stark effect. The electrostatic Stark effect appeared with a Stark
tuning rate of Δμ = 0.64 ± 0.04 cm–1/(108 V·m–1) for CO in the ASW
matrix. The charge transfer effect on the frequency change had a sensitivity
factor of Δν̅/σ ≈ 200 cm–1/C·m–2 for chemisorbed CO on Pt(111), where
σ is the excess charge density of the Pt surface. From these
observations, we suggest that in electrochemical experiments, where
ν(C–O) of CO adsorbates on the electrode surface changes
with the electrode bias potential, the frequency shift may result
predominantly from the metal–adsorbate charge transfer rather
than the electrostatic Stark shift.
Brute force orientation by an electric field is a promising way of controlling the orientation of polar molecules in the gas phase, but its application to condensed-phase molecules has been very limited. We studied the reorientation of formaldehyde molecules in a solid Ar matrix under the influence of a strong electric field using reflection absorption infrared spectroscopy. Asymptotically perfect alignment of the formaldehyde molecules along the field was achieved at field strengths exceeding 1×10 V m . The vibrational bands of the aligned molecules exhibited a unidirectional Stark shift proportional to the field strength. The reorientation of the molecules was reversible despite the cryogenic solid environment of the system.
The orientation state of hydrogen chloride (HCl) molecules in a solid argon matrix was reversibly controlled by applying an external electric field of up to 4 × 10 V·m using the ice film capacitor method. The rovibrational transitions of the field-oriented HCl were measured by reflection absorption infrared spectroscopy with p-polarized light. Upon application of the external field, free rotation of HCl inside the matrix gradually changed to perturbed rotation and then to a pendular state harmonically bound in the Stark potential well. Further increase in the field strength increased the degree of dipole alignment along the field direction, approaching an asymptotically perfect orientation of the molecules with an average tilt angle of <30° at a field strength above 1 × 10 V·m.
Brute force orientation by an electric field is a promising way of controlling the orientation of polar molecules in the gas phase, but its application to condensed‐phase molecules has been very limited. We studied the reorientation of formaldehyde molecules in a solid Ar matrix under the influence of a strong electric field using reflection absorption infrared spectroscopy. Asymptotically perfect alignment of the formaldehyde molecules along the field was achieved at field strengths exceeding 1×108 V m−1. The vibrational bands of the aligned molecules exhibited a unidirectional Stark shift proportional to the field strength. The reorientation of the molecules was reversible despite the cryogenic solid environment of the system.
Chemical reactions are extremely difficult to occur in ice at low temperature, where atoms and molecules are frozen in position with minimal thermal energy and entropy. Contrary to this general behavior, certain weak acids including fluoroacetic acids dissociate spontaneously and more efficiently in cryogenic ice than in aqueous solution at room temperaure. The enhanced reactivity of weak acids is an unexpected consequence of proton-transfer equilibrium in ice. The configurational entropy of protons in ice shifts the acid dissociation equilibrium forward. This configurational entropy, although a solid-state property, is comparatively large in magnitude with the entropy of vaporization and can effectively drive proton-transfer reactions in ice.
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