Transitions Of D2O Research Articles

Toward Elimination of Discrepancies between Theory and Experiment: Anharmonic Rotational-Vibrational Spectrum of Water in Solid Noble Gas Matrices.

Rotational–vibrational spectroscopy of water in solid noble gas matrices has been studied for many decades. Despite that, discrepancies persist in the literature about the assignment of specific bands. We tackle the involved rotational–vibrational spectrum of the water isotopologues H216O, HD16O, and D216O with an unprecedented combination of experimental high-resolution matrix isolation infrared (MI-IR) spectroscopy and computational anharmonic vibrational spectroscopy by vibrational configuration interaction (VCI) on high-level ab initio potential energy surfaces. With VCI, the average deviation to gas-phase experiments is reduced from >100 to ≈1 cm–1 when compared to harmonic vibrational spectra. Discrepancies between MI-IR and VCI spectra are identified as matrix effects rather than missing anharmonicity in the theoretical approach. Matrix effects are small in Ne (≈1.5 cm–1) and a bit larger in Ar (≈10 cm–1). Controversial assignments in Ne MI-IR spectra are resolved, for example, concerning the ν3 triad in HDO. We identify new transitions, for example, the ν2 101 ← 110 transition in D2O and H2O or the ν3 000 ← 101 transition in D2O, and reassign bands, for example, the band at 3718.9 cm–1 that is newly assigned as the 110 ← 111 transition. The identification and solution of discrepancies for a well-studied benchmark system such as water prove the importance of an iterative and one-hand combination of theory and experiment in the field of high-resolution infrared spectroscopy of single molecules. As the computational costs involved in the VCI approach are reasonably low, such combined experimental/theoretical studies can be extended to molecules larger than triatomics.

A nanosecond near-infrared step-scan Fourier transform absorption spectrometer: Monitoring singlet oxygen, organic molecule triplet states, and associated thermal effects upon pulsed-laser irradiation of a photosensitizer

A step-scan Fourier transform (FT) near infrared (IR) spectrometer has been used to record nanosecond time-resolved absorption spectra of singlet molecular oxygen (a1Δg→b1Σg+) created by pulsed-laser irradiation of a photosensitizer dissolved in a solvent. In addition to oxygen’s a→b transition at ∼5200 cm−1, other laser-induced changes in absorbance can be monitored over the spectral range ∼4000–11 000 cm−1. These include transients derived from the singlet oxygen sensitizer, and examples are provided with triplet state absorption spectra of several molecules in benzene as well as water. In this report, issues pertinent to the use of a step-scan FT spectrometer to monitor weak laser-induced near-IR signals are discussed. Specifically, key modifications to a commercially available instrument are described, and the relevant sources of noise from electronic components are outlined. Transient laser-induced thermal effects such as thermal lenses and temperature-dependent shifts in solvent absorption bands can also interfere with the detection of a given signal, and these complications are likewise described. The significance of these issues is apparent when monitoring oxygen’s a→b transition in D2O, a scenario in which the present detection limits of this technique are embodied. Much of the information in this report transcends the specific problem of singlet oxygen detection, however, and addresses issues general to the use of a step-scan FT spectrometer for time-resolved IR measurements.

Light-Scattering Study of Coil-to-Globule Transition of a Poly(N-isopropylacrylamide) Chain in Deuterated Water

Using a nearly monodisperse (Mw/Mn < 1.1) high molar mass (1.3 × 107 g/mol) poly(N-isopropylacrylamide) (PNIPAM), we studied the conformation change of individual PNIPAM chains between an extended random coil and a fully collapsed thermodynamically stable single chain globule in an extremely dilute D2O solution (∼6.3 × 10-7 g/mL) by a combination of static and dynamic laser light scattering (LLS). In comparison with PNIPAM in H2O, the ϑ-temperature of PNIPAM in D2O is slightly higher, shifting from 30.6 to 32.1 °C. Moreover, the coil-to-globule transition in D2O showed no molten globular state, different from PNIPAM in H2O. However, the molten globular state exists in the globule-to-coil transition in D2O. The size of the PNIPAM chains near the ϑ-temperature in the globule-to-coil transition is smaller than that in the coil-to-globule transition; i.e., there exists a hysteresis, similar to PNIPAM in H2O. The hysteresis can be attributed to the intrachain interaction formed in the globular state. Our results indicate that the PNIPAM chains are stiffer in D2O than in H2O.

Acid-induced unfolding and refolding transitions of cytochrome c: A three-state mechanism in water and deuterium oxide

Whereas the salt-dependent conformational transition of acid-denatured horse ferricytochrome c at pH 2 is approximated by a two-state mechanism from the acid-unfolded state to the molten globule state [Kataoka, M., Hagihara, Y., Mihara, K., & Goto, Y. (1993) J. Mol. Biol. 229, 591-596], the corresponding transition in D2O has been proposed to involve a noncompact, alpha-helical intermediate state (the pre-molten globule state) [Jeng, M.-F., & Englander, S. W. (1991) J. Mol. Biol. 221, 1045-1061]. To examine the proposed difference in the conformational transitions, we carried out the HCl and DCl titrations of cytochrome c in H2O and D2O, respectively, measured by far-UV circular dichroism, tryptophan fluorescence, and Soret absorption. In both D2O and H2O, unfolding from the native state to the acid-unfolded state and subsequent refolding to the molten globule state were observed. In either solvent, the conformational transitions were well approximated by a minimal three-state mechanism consisting of the native, molten globule, and acid-unfolded states. Thus, our results did not substantiate the presence of a pre-molten globule state in D2O. Acetylation of amino groups of cytochrome c is known to stabilize the molten globule state at pH 2. On the basis of the three-state mechanism, we constructed a conformational phase diagram for the effect of pH and the degree of acetylation. This phase diagram was similar to that of the pH- and salt-dependent conformational transition of cytochrome c, suggesting that the effects of acetylation on the conformational states are similar to those of salt.

The dipole moment of water: Stark effects in D2O and HDO

The dipole moment of D2O has been determined from Stark effect measurements for the 313–220 and 441–532 microwave transitions as 1·857 ± 0·006 and 1·869 ± 0·005D respectively. A rotational dependence of dipole moment has also been established for HDO through μa in the 220–221 and 532–533 transitions; μa was determined as 0·662 ± 0·001 and 0·644 ± 0·001D respectively. The total dipole moment for HDO has been determined from the 321–414 transition to be 1·85 ± 0·01D and to lie within 0·1° of the bisector of the HOD angle. High resolution Stark spectroscopy has been performed on the 624–615 transition of D2O with improved precision using the 337 μm emission line of the HCN laser. This experiment has confirmed the dipole results from the microwave work and the frequency of the 624–615 transition in D2O has been determined as 890 395 ± 3 MHz. The slight increase of dipole moment with deuteration is consistent with the dipole moment for H2O determined from the dielectric constant. This increase is discussed for...