We present the isolated gas phase infrared spectra of formic acid dimer, (HCOOH)2, and its deuterated counterpart formic-d acid, (DCOOH)2, at room temperature. The formic acid dimer spectrum was obtained by spectral subtraction of a spectrum of formic acid vapor recorded at low pressure from that recorded at a higher pressure. The spectra of formic acid vapor contain features from both formic acid monomer and formic acid dimer, but at low and high pressures of formic acid, the equilibrium is pushed towards the monomer and dimer, respectively. A similar approach was used for the formic-d acid dimer. Building on the previous development of the Molecular Mechanics with Proton Transfer (MMPT) force field for simulating proton transfer reactions, molecular dynamics (MD) simulations were carried out to interpret the experimental spectra in the OH-stretching region. Within the framework of MMPT, a combination of symmetric single and double minimum potential energy surfaces (PESs) provides a good description of the double proton transfer PES. In a next step, potential morphing together with electronic structure calculations at the B3LYP and MP2 level of theory was used to align the computed and experimentally observed spectral features in the OH-stretching region. From this analysis, a barrier for double proton transfer between 5 and 7 kcal mol(-1) was derived, which compares with a CCSD(T)/aug-cc-pVTZ calculated barrier of 7.9 kcal mol(-1). Such a combination of experimental and computational techniques for estimating barriers for proton transfer in gas phase systems is generic and holds promise for further improved PESs and energetics of these important systems. Additional MD simulations at the semi-empirical DFTB level of theory agree quite well for the center band position but underestimate the width of the OH-stretching band.

No abstract

The Non-Covalent Interaction (NCI) index is a new topological tool that has recently been added to the theoretical chemist's arsenal. NCI fills a gap that existed within topological methods for the visualization of non-covalent interactions. Based on the electron density and its derivatives, it is able to reveal both attractive and repulsive interactions in the shape of isosurfaces, whose color code reveals the nature of the interaction. It is interesting to note that NCI can even be calculated at the promolecular level, making it a suitable tool for big systems, such as proteins or DNA. Within this chapter we will review the main characteristics of NCI, its similarities with and differences from previous approaches. Special attention will be paid to the visualization of new interaction types. Being based on the electron density, NCI is not only very stable with respect to the calculation method, but it is also a suitable tool for detecting new bonding mechanisms, since all such mechanisms should have a detectable effect on the electron density. This type of approach overcomes the limitations of bond definition, revealing all interaction types, irrespective of whether they have a name or have previously been identified. Finally, we will show how this tool can be used to understand chemical change along a chemical reaction. We will show several examples of torquoselectivity and put forward an explanation of selectivity based on secondary interactions which is complementary to the historical orbital approach.A complete NCI perspective: from new bonds to reactivity 3

No abstract

The intramolecular hydrogen bonding in methyl lactate was studied with Fourier transform infrared (FTIR) spectroscopy, intracavity laser photoacoustic spectroscopy, and cavity ring-down spectroscopy. Vapor phase spectra were recorded in the ΔvOH = 1-4 OH-stretching regions, and the observed OH-stretching transitions were compared with theoretical results. Transition frequencies and oscillator strengths were obtained using a one-dimensional anharmonic oscillator local mode model with potential energy and dipole moment surfaces calculated at the CCSD(T)-F12a/VDZ-F12 level. The three most abundant conformers of methyl lactate all appear to possess an intramolecular hydrogen bond, with the hydroxyl group forming a hydrogen bond with either the carbonyl or ester oxygen. The intramolecular hydrogen bonds were investigated theoretically by analyses based on electron density topology, natural bond orbital analysis, and visualization of the electrostatic potential energy.

We have measured the infrared spectra of ethanol·dimethylamine and methanol·dimethylamine complexes in the 299-374 K temperature range, and have determined the enthalpy of complex formation (ΔH) to be -31.1 ± 2 and -29.5 ± 2 kJ mol(-1), respectively. The corresponding values of the Gibbs free energy (ΔG) are determined from the experimental integrated absorbance and a calculated oscillator strength of the OH-stretching vibrational transition to be 4.1 ± 0.3 and 3.9 ± 0.3 kJ mol(-1) at 302 and 300 K, respectively. The entropy, ΔS is determined from the values of ΔH and ΔG to be -117 ± 7 and -111 ± 10 J (mol K)(-1) for the ethanol·dimethylamine and methanol·dimethylamine complexes, respectively. The determined ΔH, ΔG and ΔS values of the two complexes are similar, as expected by the similarity to their donor molecules ethanol and methanol. Values of ΔH, ΔG and ΔS in chemical reactions are often obtained from quantum chemical calculations. However, these calculated values have limited accuracy and large variations are found using different methods. The accuracy of the present ΔH, ΔG and ΔS values is such that the benchmarking of theoretical methods is possible.

Accurate determination of the atmospheric abundance of hydrogen bound bimolecular complexes is necessary, as hydrogen bonds are partly responsible for the formation and growth of aerosol particles. The abundance of a complex is related to the Gibbs free energy of complex formation (∆G), which is often obtained from quantum chemical calculations that rely on calculated values of the enthalpy (∆H) and entropy (∆S) of complex formation. However, calculations of ∆H and in particular ∆S are associated with large uncertainties, and accurate experimental values are therefore crucial for theoretical benchmarking studies. Infrared measurements of gas phase hydrogen bound complexes were performed in the 300 to 373 K range, and lead to a purely experimental determination of ∆H using the van't Hoff equation. Equilibrium constants were determined by combining an experimental and calculated OH-stretching intensity, from which values of ∆G and hence ∆S could be determined. Thus we can determine ∆G, ∆H and ∆S for a bimolecular complex. We find that in the 300 to 373 K temperature range the determined ∆H and ∆S values are independent of temperature.