Basil Curnutte

Far-Infrared Spectrum of Liquid Water*

The infrared spectra of H2O and D2O in the liquid state at ambient temperature (30°C) have been remapped in the spectral region between 10 and 330 μ. The major features observed were extremely intense absorption bands with maxima at 685 and 505 cm−1 in H2O and D2O, respectively. These major bands are overlapped at the low-frequency ends by much less intense bands producing transmittance minima near 193 and 187 cm−1, respectively. No evidence was obtained for the series of narrow bands recently reported by Stanevich and Yaroslavskii. Extinction coefficients have been determined for the range 170–50 cm−1 and are compared with recent data; present data on linear absorption coefficients for H2O in the range 1500–1100 cm−1 are in fair agreement with the results of previous workers. The influence of temperature variations on the frequencies of infrared bands has been studied for all bands in the region between 4000 and 32 cm−1. Theoretical interpretation of the results is discussed briefly.

Refractive Index of Water in the Infrared

The infrared reflectance of water at incidence angles of 70° and 75° has been measured in the spectral region between 2 and 25 μm. We have used the measured reflectances of polarized radiant flux to determine the real part of the refractive index from the Fresnel equations. The imaginary part of the refractive index can be accurately determined from reflection measurements alone only in regions of strong absorption; in other regions we have used values based on measurements of transmittance. We give values of the optical constants in tabular form and compare them with recent determinations by other investigators.

Optical constants of solid ammonia in the infrared

On the basis of transmission measurements of samples at temperatures near the melting point, we have determined the Lambert absorption coefficient α(ν) for solid ammonia in the spectral range 950–5300 cm−1. The imaginary part k of the complex index of refraction Nˆ=n+ik can be obtained from the defining relation k(ν) = λα(ν)/4π. Using the measured values of α(ν), we have employed subtractive Kramers–Kronig analysis to obtain the real part n(ν) of Nˆ in the range 960–5300 cm−1. In this analysis, we have used a computed value n(ν) at 4800 cm−1 based on a Lorentz–Lorenz relation and the value of n(ν) for liquid ammonia that we measured previously. The resulting optical constants k(ν) and n(ν) for solid ammonia are presented graphically and tabularly.

The infra-red spectrum of water

The Lambert absorption coefficient of water has been measured in the far infra-red spectral region 800 to 50 cm-1. The results, along with earlier measurements in the near infra-red, provide values of the imaginary part k of the refractive index over the spectral range 4400 to 50 cm-1. Kramers-Kronig techniques have been used to obtain values of the corresponding real part n of the refractive index over this spectral range. The values of these constants provide a complete quantitative description of the optical properties of water in the infra-red.

Water structure and the denaturation of DNA.

Nuclear magnetic resonance measurements of the water hydroxyl proton absorption area, line width and chemical shift were made to investigate the binding of water to the DNA molecule. These measurements covered a temperature range of 10 to 90°C, which included the denaturation temperature of DNA in all cases. The results indicate that the amount of water bound to DNA is very small, and is not measureably changed during denaturation. Studies of the correlation of the denaturation temperature of DNA with the structure of water and D 2 O were measured by the chemical shift of the hydroxyl proton resonance. If the chemical shift in sodium perchlorate solutions is divided into a contribution from the Na + and a contribution from the ClO 4 − , then the denaturation temperature of DNA is exactly correlated with the chemical shift due to the ion ClO 4 − . Both Na + and ClO 4 − shift the proton resonance in water to higher fields; however, the Na + has an ordering effect on the water, through hydration, while the ClO 4 − is shown to disorder water in a manner similar to an increase of temperature. The difference in the effect of Na + and ClO 4 − makes clear the difficulty in using nuclear magnetic resonance as a tool in studying changes of water structure in biological systems. The same measurement may reflect two totally different structural changes, one of biological significance, the other not. The denaturation of DNA is discussed in terms of the structure of the surrounding water. It is postulated that intra-water hydrogen bonding supports the secondary structure of DNA, thereby preventing base pairs from separating when intra-strand hydrogen bonds are broken. The probability of these bonds reforming is thus increased.

Collisional broadening of infra-red absorption lines

The Lorentz half-widths of collision-broadened lines in the rotation-vibration bands of diatomic molecules vary with line number |m| in the P and R branches. The observed variation of half-width for lines in the 0 → 1 and 0 → 2 bands of CO and HCl are interpreted in terms of a simple fitting procedure. One dominant source of line broadening is assumed to consist of diabatic hard collisions involving transitions from each rotational level to all higher rotational levels; the effectiveness of this process, which varies from line to line, is described in terms of an empirically adjusted collision cross section, a maximum collision parameter related to independently measured molecular properties, and upon the availability of the required energy and angular momentum in molecular collisions. The second source of line broadening, assumed to be the same for all lines, includes all other types of collisions and is represented by a single empirically adjusted cross section. The simple fitting procedure is applied s...

A normal coordinate analysis based on the local structure of liquid water

Abstract Recent far-infrared, Raman, hyper-Raman, and inelastic neutron scattering measurements in the frequency range of about 100 to 1000 cm −1 have provided data related to the intermolecular motions of liquid water. A normal coordinate analysis of a simple mathematical model is used to study the motions of a rigid water molecule hydrogen bonded to its four nearest neighbors. The potential energy function for this model contains essentially four adjustable parameters, independent of temperature. Three of the parameters represent the stretching and bending of the hydrogen bonds. The fourth parameter is a term representing the interaction of the electric dipole moment of the water molecule with the local electric field. This four parameter model reproduces the frequencies and assignments of Walrafen using two bending constants which differ by about five percent. The number of parameters may be reduced to three with only small changes in the calculated frequencies if the bending constants are set equal. Both the hydrogen bonds and the large electric dipole moment of the molecule play important roles in the anomalous behavior found in many of the physical properties of water. The broad absorption bands found in this frequency range are incorporated in the model by means of a distribution of 0-0 distances based on the X-ray diffraction experiments of Narten, Danford, and Levy. The hydrogen bond properties are represented by the empirical potential function of Lippincott and Schroeder. The far infrared intensity ratios were calculated using a generalization of a dipole moment function used by Phillips. The results lend support to the frequency assignments of Walrafen. They also may be used to explain the experimental far-infrared spectra of Draegert et al. in terms of bandwidth, isotopic substitution, and temperature dependence. In addition, far-infrared frequencies are calculated with this model for ice which agree well with the measured frequency peaks.

The intramolecular vibrations of the water molecule in the liquid state

Abstract A normal coordinate treatment of a general bent XYZ molecule and a continuum picture of the liquid state are used to calculate the frequency distribution of intramolecular vibrations of a water molecule in the liquid state. The effect of neighboring molecules on the vibrational frequencies is taken into account by assuming complete tetrahedral hydrogen bonding with a distribution of equilibrium OO distances. The equilibrium OO distance distributions at various temperatures are obtained from X-ray pair correlation functions. The vibrational distributions are converted into spectra by taking the intensity of scattering or absorption of the band at each frequency as being determined by the number of oscillators having that particular frequency at any instant. Of the four adjustable parameters in the potential energy function, the stretching constants are chosen from the Lippincott-Schroeder potential function, while the remaining three are determined from H 2 O, HDO, and D 2 O infrared vapor data.

Temperature dependence of the Raman and infrared spectrum of liquid formic acid

Abstract The shift of the Raman and infrared bands of liquid formic acid were determined for a temperature change from 10°C to 80°C. These data together with previous data from the literature on other phases of formic acid were used to make assignments of the fundamental vibrational frequencies of liquid formic acid and can be explained by the existence of a helical-chain polymer in the liquid. The liquid phase assignments improve on the low frequency assignments for the vapor dimer.

A local-structure model for calculation of lattice vibrations in liquid water

Abstract The local-structure model of Bryan and Curnutte, which treated a single molecule in a rigid cage, has been extended to include a central water molecule and its four nearest neighbors surrounded by a rigid cage of next-nearest neighbors. The influence of the next-nearest neighbors is accounted for by the average forces they exert on the five-molecule local-structure group. With parameters based on water at 25°C, we give calculations of the spectra of liquid H 2 O and D 2 O at 25°C and for ice I . The results of the calculations are compared with observed spectra and with recent molecular-dynamics calculations.