The Role of Anharmonicity in Hydrogen-Bonded Systems: The Case of Water Clusters

The nature of vibrational anharmonicity has been examined for the case of small water clusters using second-order vibrational perturbation theory (VPT2) applied on second-order Møller–Plesset perturbation theory (MP2) potential energy surfaces. Using a training set of 16 water clusters (H2O)n=2–6,8,9 with a total of 723 vibrational modes, we determined scaling factors that map the harmonic frequencies onto anharmonic ones. The intermolecular modes were found to be substantially more anharmonic than intramolecular bending and stretching modes. Due to the varying levels of anharmonicity of the intermolecular and intramolecular modes, different frequency scaling factors for each region were necessary to achieve the highest accuracy. Furthermore, new scaling factors for zero-point vibrational energies (ZPVE) and vibrational corrections to the enthalpy (ΔHvib) and the entropy (Svib) have been determined. All the scaling factors reported in this study are different from previous works in that they are intended for hydrogen-bonded systems, while others were built using experimental frequencies of covalently bonded systems. An application of our scaling factors to the vibrational frequencies of water dimer and thermodynamic functions of 11 larger water clusters highlights the importance of anharmonic effects in hydrogen-bonded systems.

The Role of Anharmonicity in Hydrogen-Bonded Systems: The Case of Water Clusters

The nature of vibrational anharmonicity has been examined for the case of small water clusters using second-order vibrational perturbation theory (VPT2) applied on second-order Møller–Plesset perturbation theory (MP2) potential energy surfaces. Using a training set of 16 water clusters (H2O)n=2–6,8,9 with a total of 723 vibrational modes, we determined scaling factors that map the harmonic frequencies onto anharmonic ones. The intermolecular modes were found to be substantially more anharmonic than intramolecular bending and stretching modes. Due to the varying levels of anharmonicity of the intermolecular and intramolecular modes, different frequency scaling factors for each region were necessary to achieve the highest accuracy. Furthermore, new scaling factors for zero-point vibrational energies (ZPVE) and vibrational corrections to the enthalpy (ΔHvib) and the entropy (Svib) have been determined. All the scaling factors reported in this study are different from previous works in that they are intended for hydrogen-bonded systems, while others were built using experimental frequencies of covalently bonded systems. An application of our scaling factors to the vibrational frequencies of water dimer and thermodynamic functions of 11 larger water clusters highlights the importance of anharmonic effects in hydrogen-bonded systems.

Exploration of the Potential Energy Surfaces, Prediction of Atmospheric Concentrations, and Prediction of Vibrational Spectra for the HO2···(H2O)n (n = 1−2) Hydrogen Bonded Complexes

The hydroperoxy radical (HO2) plays a critical role in Earth's atmospheric chemistry as a component of many important reactions. The self-reaction of hydroperoxy radicals in the gas phase is strongly affected by the presence of water vapor. In this work, we explore the potential energy surfaces of hydroperoxy radicals hydrogen bonded to one or two water molecules, and predict atmospheric concentrations and vibrational spectra of these complexes. We predict that when the HO2 concentration is on the order of 108molecules·cm-3 at 298 K, that the number of HO2···H2O complexes is on the order of 107molecules·cm-3 and the number of HO2···(H2O)2 complexes is on the order of 106molecules·cm-3. Using the computed abundance of HO2···H2O, we predict that, at 298 K, the bimolecular rate constant for HO2···H2O + HO2 is about 10 times that for HO2 + HO2.

Ability of the PM3 Quantum Mechanical Method to Model Intermolecular Hydrogen Bonding between Neutral Molecules

The PM3 semiempirical quantum-mechanical method was found to systematically describe intermolecular hydrogen bonding in small polar molecules. PM3 shows charge transfer from the donor to acceptor molecules on the order of 0.02-0.06 units of charge when strong hydrogen bonds are formed. The PM3 method is predictive; calculated hydrogen bond energies with an absolute magnitude greater than 2 kcal mol-' suggest that the global minimum is a hydrogen bonded complex; absolute energies less than 2 kcal mol-' imply that other van der Waals complexes are more stable. The geometries of the PM3 hydrogen bonded complexes agree with high-resolution spectroscopic observations, gas electron diffraction data, and high-level ab initio calculations. The main limitations in the PM3 method are the underestimation of hydrogen bond lengths by 0.1-0.2 for some systems and the underestimation of reliable experimental hydrogen bond energies by approximately 1-2 kcal mol-l. The PM3 method predicts that ammonia is a good hydrogen bond acceptor and a poor hydrogen donor when interacting with neutral molecules. Electronegativity differences between F, N, and 0 predict that donor strength follows the order F > 0 > N and acceptor strength follows the order N > 0 > F. In the calculations presented in this article, the PM3 method mirrors these electronegativity differences, predicting the F-H- - -N bond to be the strongest and the N-H- - -F bond the weakest. It appears that the PM3 Hamiltonian is able to model hydrogen bonding because of the reduction of two-center repulsive forces brought about by the parameterization of the Gaussian core-core interactions. The ability of the PM3 method to model intermolecular hydrogen bonding means reasonably accurate quantum-mechanical calculations can be applied to small biologic systems.

Perception of non-invasive ventilation in adult Swiss intensive care units

The real utilisation scenario of non-invasive ventilation (NIV) in Swiss ICUs has never been reported. Using a survey methodology, we developed a questionnaire sent to the directors of the 79 adult ICUs to identify the perceived pattern of NIV utilisation. We obtained a response rate of 62%. The overall utilisation rate for NIV was 26% of all mechanical ventilations, but we found significant differences in the utilisation rates among different linguistic areas, ranging from 20% in the German part to 48% in the French part (p <0.01). NIV was mainly indicated for the acute exacerbations of COPD (AeCOPD), acute cardiogenic pulmonary edema (ACPE) and acute respiratory failure (ARF) in selected do-not-intubate patients. In ACPE, CPAP was much less used than bi-level ventilation and was still applied in AeCOPD. The first line interface was a facial mask (81%) and the preferred type of ventilator was an ICU machine with an NIV module (69%). The perceived use of NIV is generally high in Switzerland, but regional variations are remarkable. The indications of NIV use are in accordance with international guidelines. A high percentage of units consider selected do-not-intubate conditions as an important additional indication.

Compliance and knowledge about glaucoma in patients at tertiary glaucoma units

To document the rate of self-reported compliance and glaucoma-related knowledge in Swiss patients and to identify risk factors for their poor compliance. This was an observational study, including a total of 200 consecutive patients already under glaucoma medication in two Swiss tertiary glaucoma clinics (Geneva and Bern). Personal characteristics, presence of systemic disease, compliance with glaucoma medication, attitude to the ophthalmologist, and glaucoma-related attitudes were ascertained by means of a predetermined questionnaire with 40 questions. Patients were subsequently assessed for the ability to correctly instil placebo eye drops. Non-compliance with glaucoma medication was defined as omitting more than two doses a week as reported by the patient. Logistic regression was used to evaluate how patient characteristics and knowledge about the disease were related to compliance. Overall, 81% (n = 162) of patients reported to be compliant. Forgetfulness was the most frequently cited reason for non-compliance with dosing regimen (63%). Although 90.5% (n = 181) of patients believed glaucoma medication to be efficient, only 28% (n = 56) could correctly define glaucoma. Factors positively associated with compliance were 'knowledge of glaucoma' [adjusted odds ratio (OR) 4.77 (95% CI 1.36-16.70)] and 'getting help for administration of drops' [OR 2.95 (1.25-6.94)]. These findings indicate that despite the comparatively high compliance rate among glaucoma patients, knowledge of glaucoma remains poor in long-term glaucoma sufferers. Improving knowledge about the disease is important since it is positively associated with compliance in our study.