On the relation between basis set convergence and electron correlation: a critical test for modern ab initio quantum chemistry on a "mindless” data set

The correlation of the only two error sources in the solution of the electronic Schrödinger equation is addressed: the basis set convergence (incompleteness) error (BSIE) and the electron correlation effect. The electron correlation effect and basis set incompleteness error are found to be correlated for all of the molecules in Grimme's "mindless” data set (MB08-165). One can use an extrapolation to the HF or MP2 complete basis set (CBS) limit to see with which type of quantum chemical problem ("simple” and "hard”) the researcher is dealing. The origin of the slow convergence of the partial wave expansion can be the Kato cusp condition for electron-electron coalescence. Such an extrapolation is possible for many large molecular systems and would give the researcher an idea about the expected electron correlation level that would lead to the desired theoretical accuracy. In other words, it is possible to use not only the CBS energy value itself but the speed with which it is reached to get extra information about the molecular system under stud

Fragmentation of benzylpyridinium "thermometer” ions and its effect on the accuracy of internal energy calibration

Electrospray ionization mass spectrometry (ESI-MS) is a powerful analytical method to study biomolecules and noncovalent complexes. The prerequisite for their intact observation is soft ionization. In ESI, the internal energy of ions is primarily influenced by collisional activation in the source. The survival yield method is frequently used to probe the energy deposition in ions during the electrospray process. In the present work, we investigate the fragmentation pathways of para-substituted benzylpyridinium ions, the most widely used "thermometer ions” in the survival yield method. In addition to the C-N bond cleavage, alternative fragmentation channels were found for the compounds studied. We consider these pathways to result from intramolecular rearrangements. The effect of these additional fragments on the accuracy of the internal energy calibration is estimated for both collision-cell and in-source collision-induced dissociation (CID). Altogether, results presented suggest that a correction of the energy scale is necessary for the method based on benzylpyridinium ions to precisely quantify ion internal energie

What Happens to Hydrophobic Interactions during Transfer from the Solution to the Gas Phase? The Case of Electrospray-Based Soft Ionization Methods

The disappearance of the hydrophobic effect in the gas phase due to the absence of an aqueous surrounding raises a long-standing question: can noncovalent complexes that are exclusively bound by hydrophobic interactions in solution be preserved in the gas phase? Some reports of successful detection by mass spectrometry of complexes largely stabilized by hydrophobic effect are questionable by the presence of electrostatic forces that hold them together in the gas phase. Here, we report on the MS-based analysis of model supramolecular complexes with a purely hydrophobic association in solution, β-cyclodextrin, and synthetic adamantyl-containing ligands with several binding sites. The stability of these complexes in the gas phase is investigated by quantum chemical methods (DFT-M06). Compared with the free interaction partners, the inclusion complex between β-cyclodextrin and adamantyl-containing ligand is shown to be stabilized in the gas phase by ΔG = 9.6kcal mol-1. The host-guest association is mainly enthalpy-driven due to strong dispersion interactions caused by a large nonpolar interface and a high steric complementarity of the binding partners. Interference from other types of noncovalent binding forces is virtually absent. The complexes are successfully detected via electrospray ionization mass spectrometry, although a high dissociation yield is also observed. We attribute this pronounced dissociation of the complexes to the collisional activation of ions in the atmospheric interface of mass spectrometer. The comparison of several electrospray-based ionization methods reveals that cold spray ionization provides the softest ion generation conditions for these complexe

Simultaneous sampling of volatile and non-volatile analytes in beer for fast fingerprinting by extractive electrospray ionization mass spectrometry

By gently bubbling nitrogen gas through beer, an effervescent beverage, both volatile and non-volatile compounds can be simultaneously sampled in the form of aerosol. This allows for fast (within seconds) fingerprinting by extractive electrospray ionization mass spectrometry (EESI-MS) in both negative and positive ion mode, without the need for any sample pre-treatment such as degassing and dilution. Trace analytes such as volatile esters (e.g., ethyl acetate and isoamyl acetate), free fatty acids (e.g., caproic acid, caprylic acid, and capric acid), semi/non-volatile organic/inorganic acids (e.g., lactic acid), and various amino acids, commonly present in beer at the low parts per million or at sub-ppm levels, were detected and identified based on tandem MS data. Furthermore, the appearance of solvent cluster ions in the mass spectra gives insight into the sampling and ionization mechanisms: aerosol droplets containing semi/non-volatile substances are thought to be generated via bubble bursting at the surface of the liquid; these neutral aerosol droplets then collide with the charged primary electrospray ionization droplets, followed by analyte extraction, desolvation, ionization, and MS detection. With principal component analysis, several beer samples were successfully differentiated. Therefore, the present study successfully extends the applicability of EESI-MS to the direct analysis of complex liquid samples with high gas content. Figure By gently bubbling nitrogen gas through beer, both volatile and non-volatile compounds can be simultaneously sampled in the form of aerosol for further analysis, allowing fast chemically fingerprinting using extractive electrospray ionization mass spectrometry (EESI-MS

THE TWO MISSING CONFORMERS OF GAS-PHASE ALANINE: A JET-COOLED RAMAN SPECTROSCOPY STUDY

Author Institution: Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, SwitzerlandThe jet-cooled spontaneous Raman spectrum of an amino acid ?-- alanine (Ala, 2-aminopropanoic acid; H$_2$NCH(CH$_3$)COOH) ?-- is reported. The low-frequency vibrational spectrum (below 500~cm$^{-1}$) was recorded and assigned using quantum chemical data: \textit{ab initio} (MP2) and density functional theory (DFT; BLYP, B3LYP, and PBE0/PBE1PBE). Band polarization measurements were used to confirm the vibrational assignments. The acquired medium resolution spectra (HWHM of approximately 4~cm$^{-1}$) allow the different alanine conformations to be distinguished. Four alanine conformers were observed and identified: two previously reported by microwave (MW) spectroscopy studies and two that were previously unreported. A set of reasons for why these conformers eluded previous studies are discussed. Selective collisional relaxation processes in the jet (associated with low interconversion barriers between different alanine conformations) that depopulate the high-energy conformers were experimentally demonstrated. Conclusions about conformational equilibrium in peptide/protein building block are made

CONFORMATIONAL EQUILIBRIUM IN GLYCINE: EXPERIMENTAL JET-COOLED RAMAN SPECTRUM

Author Institution: Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, SwitzerlandThe first observation of a gas-phase spontaneous Raman spectrum of an amino acid ?-- glycine (Gly, H$_2$NCH$_2$COOH) ?-- is reported. A molecular beam source was combined with a high sensitivity Raman setup to record a low-frequency (below 500~cm$^{-1}$) vibrational spectrum. \textit{Ab initio\it} (first principles; MP2) and density functional theory (DFT; BLYP, B3LYP, and PBE0/PBE1PBE) calculations with the aug-cc-pVTZ Dunning-type basis set were used to support vibrational assignments. Jet-cooling of glycine molecules was observed by varying the laser-nozzle distance. The acquired medium resolution spectra (4~cm$^{-1}$) allow the different glycine conformations to be distinguished. The structures of the two most stable glycine conformers, previously observed by electron diffraction and microwave spectroscopy, have been confirmed. Evidence is reported for a third conformation (\textit{gauche}-\textit{trans}-\textit{trans}), which has not been previously experimentally reported. Band polarization measurements were used to confirm the vibrational assignments

POLAR (ACYCLIC) ISOMER OF FORMIC ACID DIMER: RAMAN SPECTROSCOPY STUDY

Author Institution: Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, SwitzerlandFormic (methanoic) acid spectral range of 575--1150 cm$^{-1}$ has been studied by gas-phase Raman spectroscopy method. A weak Raman-active vibration of polar (acyclic) HCOOH dimer has been found at 864$\pm$2.1 cm$^{-1}$ and assigned using quantum chemistry data. The temperature-dependence of intensity ratios of Raman lines was used to evaluate the thermodynamic parameters of polar dimer. Its experimental dimerization enthalpy ($\bigtriangleup$H) was found to be -8.6$\pm$0.2 kcal mol$^{-1}$. Entropy of dimerization has been evaluated using theoretical (MP2) Raman scattering activities. Its value ($\bigtriangleup$S) is estimated as -36$\pm$2 cal mol$^{-1}$ K$^{-1}$. The results are compared with the published experimental data and calculations. The presented results can be used for molecular dynamics simulations, hydrogen bond energy estimation, and analysis of CH$_2$O$_2$ vapor density measurements

On the relation between basis set convergence and electron correlation: A critical test for modern ab initio quantum chemistry on a “mindless” data set

ISSN:1040-0400ISSN:1572-900

Conformational Equilibrium in Glycine: Experimental Jet-Cooled Raman Spectrum

The first observation of a gas-phase spontaneous Raman spectrum of an amino acid, glycine (Gly, H₂NCH₂COOH), is reported. A molecular beam source was combined with a high-sensitivity Raman setup to record a low-frequency (below 500 cm⁻¹) vibrational spectrum. Ab initio (MP2) and density functional theory (DFT) (BLYP, B3LYP, and PBE0) calculations with the aug-cc-pVTZ basis set were used to support vibrational assignments. Jet-cooling of glycine molecules was observed by varying the laser-nozzle distance. The acquired medium resolution spectra (4 cm⁻¹) allow the different glycine conformations to be distinguished. The structures of the two most stable glycine conformers, previously observed by electron diffraction and microwave spectroscopy, have been confirmed. Evidence is reported for a third conformation (<i>gauche</i>−<i>trans</i>−<i>trans</i>), which has not been previously experimentally reported. Band polarization measurements were used to confirm the vibrational assignments

Fragmentation of Benzylpyridinium "Thermometer" Ions and Its Effect on the Accuracy of Internal Energy Calibration

ISSN:1879-1123ISSN:1044-030