Monohydrated Clusters Research Articles

Effective Fragment Potentials for Microsolvated Excited and Anionic States.

The effective fragment potential (EFP) approach is a sophisticated hybrid approach that allows the inclusion of solvation effects when describing properties and reactivity in the condensed phase, without using empirical parameters. This work examines the performance of the EFP method when describing microsolvation in electronically excited states of neutrals and anions. The examples selected include both localized valence states, as well as diffuse nonvalence states, which represent greater challenges to conventional electronic structure methods. The equation-of-motion coupled cluster with singles and doubles (EOM-XX-CCSD) methodology has been used to provide the quantum chemical description of both the full microsolvated clusters, and the chromophoric moiety in mixed quantum/EFP calculations. We find that, when averaging over multiple configurations of microsolvated clusters, the differences between QM/EFP and full quantum results are minimal, although individual configurations often have larger errors. As expected, diffuse states have somewhat larger errors, although not significantly so. The close proximity of states leading to mixing can make QM/EFP less accurate because a change of ordering of states can occur. Other properties, such as photoelectron images and lifetimes of metastable states, are very well described for the monohydrated clusters investigated.

Structural confirmation and spectroscopic signature of N-Allyl-2‑hydroxy-5-methyl-3-oxo-2, 3-dihydrobenzofuran-2-carboxamide and its monohydrate cluster

We performed optimization of a bio-relevant molecule N-Allyl-2‑hydroxy-5-methyl-3-oxo-2, 3-dihydrobenzofuran-2-carboxamide and its monohydrate cluster in the isolated form. Potential energy scanning for the target molecule was carried out. Consequently, three stable conformers were obtained. The effect of interfusion of a water molecule on energy and vibrational modes of the target molecule was also investigated in the most stable conformer. The electronic structures and vibrational spectra of all the three conformers were computed. The FTIR spectrum of the target molecule was recorded at the spectral width of 4000–400cm−1 which was compared with the theoretically computed spectrum of the most stable conformer. We have computed Raman spectra of all the conformers. All theoretical calculations performed in this investigation were done at DFT/B3LYP level of theory. The standard normal coordinate analysis (NCA) method was used to calculate the potential energy distributions of normal modes of the target molecule and its monohydrate cluster. Moreover, the NBO calculations for the target molecule and its monohydrate cluster were done to determine electronic structures, bond energies, occupancies, HOMO-LUMO and hyper-conjugative interaction energies of donor-acceptor interactions. The enthalpy of formation showed that the formation of the monohydrated cluster is of an exothermic nature.

Experimental/Computational Study on the Impact of Fluorine on the Structure and Noncovalent Interactions in the Monohydrated Cluster of ortho-Fluorinated 2-Phenylethylamine.

Fluorine-containing medicinal compounds frequently allow modulation of physical-chemical properties. Here, we address the effect of fluorine, near the ethylamino side chain, on conformational flexibility and noncovalent interactions (NCIs) of the selected jet-cooled monohydrated cluster of 2-(2-fluoro-phenyl)-ethylamine (2-FPEA) by mass-selected resonance-enhanced two-photon ionization and ionization-loss stimulated Raman spectroscopies. Our results show that Raman spectral signatures of the 2-FPEA-H2O cluster match the scaled harmonic vibrational Raman frequencies, resulting from density functional theory calculations of the most stable 2-FPEA gauche conformer hydrogen-bonded (HB) to water, confirming the three-dimensional cluster structure. This predicted electronic structure, together with NCI analysis, allows visualization and assessment of the attractive and repulsive interactions. The comparison of the NCIs and revealed red (O-H and N-H stretches) and blue shifts (C-H stretches and CH2 out-of-plane bends) of the cluster to other class members confirm O-H···N, N-H···π, C-H···O, and C-H···F HB formation and their contribution to structure stabilization, uncovering the potential of the approach.

Unraveling hydridic-to-protonic dihydrogen bond predominance in monohydrated dodecaborate clusters.

Hydridic-to-protonic dihydrogen bonds (DHBs) are involved in comprehensive structural and energetic evolution, and significantly affect reactivity and selectivity in solution and solid states. Grand challenges exist in understanding DHBs’ bonding nature and strength, and how to harness DHBs. Herein we launched a combined photoelectron spectroscopy and multiscale theoretical investigation using monohydrated closo-dodecaborate clusters B12X122−·H2O (X = H, F, I) to address such challenges. For the first time, a consistent and unambiguous picture is unraveled demonstrating that B–H⋯H–O DHBs are superior to the conventional B–X⋯H–O HBs, being 1.15 and 4.61 kcal mol−1 stronger than those with X = F and I, respectively. Energy decomposition analyses reveal that induction and dispersion terms make pronounced contributions resulting in a stronger B–H⋯H–O DHB. These findings call out more attention to the prominent roles of DHBs in water environments and pave the way for efficient and eco-friendly catalytic dihydrogen production based on optimized hydridic-to-protonic interactions.

Stepwise Microhydration of Isoxazole: Infrared Spectroscopy of Isoxazole-(Water)n≤2 Clusters in Helium Nanodroplets.

Hydration of heterocyclic molecules plays a crucial role in biological and chemical recognition. Here, we present an infrared (IR) spectroscopic investigation of microhydrated, heterocyclic isoxazole molecule. The IR spectra of isoxazole-(water)n≤2 clusters are recorded using helium nanodroplet spectroscopy and are analyzed by quantum chemical calculations at the MP2/6-311++g(d,p) level. In the most abundant isoxazole-water dimer, the solvent water participates in a N···HO hydrogen bonding (H-bond) interaction, while in another observed structure, water simultaneously interacts with ring nitrogen and the neighboring CH group via N···HO and CH···O H-bonds. The addition of another water molecule to the monohydrated cluster results in the formation of a single isomer that features a seven-membered ring, in which the water dimer simultaneously interacts with skeletal nitrogen and the adjacent CH group through N···HO and CH···O bonds.

Evaluation of Ar tagging toward the vibrational spectra and zero point energy of X-HOH, X-DOH, and X-HOD, for X = F, Cl, Br.

In this study, we theoretically evaluated the effect of argon tagging toward the binding energy and vibrational spectra of water halide anion complexes Ar.X-HOH, Ar.X-HOD, and Ar.X-DOH (X = F, Cl, Br). The ionic hydrogen bond (IHB) OH stretching mode was calculated to have a strong peak in the vibrational spectra, and coupling to intermolecular modes as well as bending modes was observed as combination bands and Fermi resonances. We found that the argon tagging affected the IHB OH stretching peak position in Ar.F-H2O, but not in Ar.Cl-H2O and Ar.Br-H2O. Furthermore, D-binding is favored for Cl and Br based on zero point energies, but for F our calculated zero point energies did not show a preference between H- and D-binding. We show that the competition of the energy lowering in the zero point energy of the anharmonic IHB OH (OD) stretching mode versus the intermolecular out-of-plane IHB OH (OD) wagging mode is important for determining the preference between H- and D-binding for these monohydrated halide clusters. We also found that for X-HOD the HOD bending fundamental peak is blue shifted compared to bare HOD, but is redshifted for F-DOH.

Conformer selective monohydrated clusters of 1,2,3,4 –tetrahydroisoquinoline in S0: I-Potential energy surface studies, vibrational signatures and NBO analysis

The global minima in the potential energy surface of 1, 2, 3, 4-tetrahydroisoquinoline (THIQ):(H2O) prefer the twisted equatorial orientation of the NH group. It is characterized by a strong OH⋅⋅⋅N and a weak O-H⋅⋅⋅π hydrogen bonds. The computations at the ab initio level and DFT methods with M06–2X and ωB97X-D functionals generated consistent results. The monohydrate with the axial orientation of NH having a single OH⋅⋅⋅N bond in the twisted configuration of THIQ is higher by 240 ± 40 cm−1. The N⋅⋅⋅H bond length is smaller in the latter. All the clusters are located in the potential energy surface of THIQ :(H2O) drawn with the variation of the dihedral angles of the saturated ring of THIQ. Various kinds of hydrogen bonds are observed. Binding energy of each structure is computed incorporating BSSE corrections. Interestingly, the largest of them correspond to a conformer of THIQ, where it was a transition state in the bare molecule. The vibrational analysis of the clusters indicates the reassignment of a number of bands in the observed IR spectra. The red-shifts observed in the different stretching modes of water in the two lowest energy clusters are suitably explained. This is further verified from NBO computations. The changes in s-character, NBO energies and OH bond lengths with the variation of hydrogen bond distances clearly signify the dominant nature of hyperconjugation mechanism. The natures of each OH bonds of water are investigated thoroughly. The bond which is not taking part in hydrogen bonding shows some apparently unexpected behaviors. HOMO and LUMO plots and their energy differences are employed to qualitatively explain the experimental results observed earlier. Changes in Molecular Electrostatic Potentials (MEP) on cluster formation are explained and confirm our computations. Maximum hardness principle (MHP) is followed by the most stable monohydrate only. Hardness values display a certain arrangement depending on the type of hydrogen bonding sites. We suggest further experiments to confirm our key observations.

Conformational preferences of monohydrated clusters of imidazole derivatives revisited.

We present the IR spectroscopic investigations of benzimidazole (BIM), N-methylbenzimidazole (MBIM), and their monohydrates (W1) carried out in a supersonic jet in the region of N-H, C-H, and O-H stretching fundamentals. The C-H stretching modes in the monomers were studied with the aim of identifying the C(2)H mode (the C atom between the two N atoms in the imidazole moiety) which is known to play an important role as a H-bond donor in enzymes and ionic liquids. The assignment was aided by quantum chemical calculations as well as the literature data for FTIR measurements in the matrix. The monohydrated clusters were investigated for a global comparison with previously reported conformations of hydrated imidazole and related derivatives in the gas phase, matrices, and He nanodroplets. The BIM-W1 complex showed the presence of three conformers; an N-H∙∙∙O bound conformer (A') and two O-H∙∙∙N bound conformers, tilted towards the phenyl side (A) and the imidazole side (B), respectively. Although both A' and B type conformers have been reported in the literature, our experiments identify a new conformer (conformer A) in the gas phase for the first time which has also been reported in crystal structures of histidine containing proteins. The binding energies of the three conformers were found to be of comparable magnitude, with the N-HO bound structure lying in between (∼0.02-0.04 kcal mol(-1)) the O-H∙∙∙N bound ones at the counterpoise corrected (cp) MP2/aug-cc-pVDZ level of theory. The formation of two distinct but closely related O-H∙∙∙N bound conformers (A and B) was additionally confirmed by studying the MBIM-W1 complex. Binding energies of the MBIM-W1 conformers were found to be higher than those of the analogous BIM-W1 conformers by 0.2 kcal mol(-1) at the cp-MP2/aug-cc-pVDZ level. The C(2)-H∙∙∙O or π bound water structures were not observed in the beam for monohydrated clusters of either monomer. While QTAIM calculations predicted secondary stabilization in the A type conformer by a C(4)-HO hydrogen bond, such an effect due to a possible C(2)-HO interaction was not found for conformer B. Experimentally, however, no spectral evidence was found for either the C(4)-H∙∙∙O or the C(2)-H∙∙∙O interaction.

Solvation Effect on the NH Stretching Vibrations of Solvated Aminopyrazine, 2-Aminopyridine, and 3-Aminopyridine Clusters

The vibrational spectra of the hydrated and methanol-solvated aminopyrazine, 2-aminopyridine and 3-aminopyridine in supersonic jets have been measured in terms of IR-UV double-resonance spectroscopy. Comparing the IR spectrum of aminopyrazine with those of 2-aminopyridine and 3-aminopyridine clusters, we determine the solvation structure of aminopyrazine to be a similar cyclic structure as hydrated 2-aminopyridine clusters [Wu, et al., Phys. Chem. Chem. Phys. 2004, 6, 515]. In the case of monohydrated aminopyrazine cluster, one of the normal modes composed of the hydrogen-bonded OH and NH stretching local modes gives the anomalously weak IR intensity, which is ascribed to the cancellation of the dipole moment change between the OH and NH stretching local modes. The solvated 3-aminopyridine clusters forms the hydrogen-bond between the pyridyl nitrogen atom and the OH group, but the amino group is indirectly affected to induce slight blue shift of the NH(2) stretches. This phenomenon is explained by inductive effect where the electron withdrawing from the amino group upon the solvation results in a "quinoid-like" structure of the amino group.

Laser Spectroscopic Study of β-Estradiol and Its Monohydrated Clusters in a Supersonic Jet

The structures of 17β-estradiol (estradiol) and its 1:1 cluster with water have been investigated in supersonic jets. The S(1)-S(0) electronic spectrum of estradiol monomer shows four strong sharp bands in the 35050-35200 cm(-1) region. Ultraviolet-ultraviolet hole-burning (UV-UV HB) and infrared-ultraviolet double-resonance (IR-UV DR) spectra of these bands indicate that they are due to four different conformers of estradiol originating from the different orientation of the OH groups in the A- and D-rings. The addition of water vapor to the sample gas generates four new bands in the 34700-34800 cm(-1) region, which are assigned to the estradiol-H(2)O 1:1 cluster with the A-ring (phenyl ring) OH acting as a hydrogen(H)-bond donor. In addition, we found very weak bands near the origin bands of bare estradiol upon the addition of water vapor. These bands are assigned to the isomers of estradiol-H(2)O 1:1 cluster having an H-bond at the D-ring OH. We determine the conformation of bare estradiol and the structures of its monohydrated clusters with the aid of density functional theory calculation and discuss the relationship between the stability of hydrated clusters and the conformation of estradiol.

Structural identification of uric acid and its monohydrates by IR-UV double resonance spectroscopy

We have employed IR-UV double resonance spectroscopy to identify the tautomeric and isomeric structures of uric acid and its monohydrated clusters which are produced by the techniques of laser-desorption and supersonic-jet cooling. The IR spectrum obtained for bare uric acid exhibits four distinct NH stretching transitions assignable to those of the most stable triketo form. We have also observed the two most stable monohydrated clusters, each with uric acid in the triketo form and water bonded to either the N3H or N9H site. It is demonstrated that the R2PI spectra of these monohydrates can be separated by using the IR-purified spectroscopic method.

Conformational Landscape of Supersonically Expanded 1‐(Fluorophenyl)ethanols and Their Monohydrated Clusters

The effects of the presence of the ring fluorine atom on the conformational landscape of supersonically expanded isomeric 1-(fluorophenyl)ethanols and their monohydrated clusters are investigated by resonant two-photon ionization (R2PI) spectroscopy, coupled with time-of-flight (TOF) mass spectrometry. In contrast to the very simple spectrum of 1-phenylethanol, the lack of structural symmetry of the aromatic rings of isomeric 1-(fluorophenyl)ethanols generates more complicated spectra, characterized by several low-frequency progressions of bands. Their interpretation is based on the strict correspondence with theoretical predictions at the D-B3LYP/6-31G** level of theory. Monohydration of the 1-(fluorophenyl)ethanol isomers favours exclusive formation of the corresponding conformers, characterized by the O-H...O(w)-H...pi intracomplex interaction and whose excitation spectrum exhibits features attributed to the C(1)-C(alpha) torsion plus intermolecular water torsion.

NQR and NMR study of hydrogen bonding interactions in anhydrous and monohydrated guanine cluster model: A computational study

In this paper extensive systematic computational study has been carried out to justify hydrogen bonding interactions and their influence on the oxygen, nitrogen and hydrogen NQR and NMR parameters of the anhydrous and monohydrated guanine crystal structures at two different levels, B3LYP and MP2, using 6-311++G** and D95** basis sets. These theoretical data have been compared with experimental NMR and NQR measurements. For further investigation, results of cluster calculation have been compared with that of a single molecule. Our theoretical NQR and NMR parameters of 17O, 15N and 2H atoms of anhydrous and monohydrated guanine exhibited extreme sensitivity to electron distribution around mentioned nuclei caused by cooperative influences of various types of hydrogen bonding interactions. Fortunately, our calculated isotropic shielding values and CS tensors for the 17O and 15N nuclei as well as obtained 14N-NQR parameters are in excellent agreement with experimental data. Therefore, we can undoubtedly conclude that for anhydrous and monohydrated guanine tetrameric clusters including intermolecular interactions, our theoretical estimates are in better agreement with observed experimental values than those in which these interactions have been ignored.

Conformational Study of Tyramine and Its Water Clusters by Laser Spectroscopy

Tyramine and its monohydrated clusters have been investigated by several laser spectroscopic methods in a pulsed molecular beam. The conformational structures and their effects on hydration have been revealed by resonant two-photon ionization (R2PI), UV-UV ion-dip, and ab initio calculations. UV rotational band contour spectra of the S1 <-- S0 origin bands enabled determination of ethylamine side chain conformations for all seven stable conformers of tyramine. When coexpanding tyramine with a mixture of Ar and water vapor, we have found two kinds of conformational effects on hydration. One is sensitive to conformation of the ethylamine chain and the other to the orientation of the OH group, particularly in the most stable pair of conformers. UV-UV ion-dip spectra detected seven stable conformers of the monohydrated clusters, of which hydrogen-bonding structures, spectral shifts, and origin band intensity distributions are well explained by considering tyramine as a hybrid of phenylethylamine (PEA) and phenol. Monohydration of the most stable gauche conformer pair (cis and trans) of tyramine leads to more detailed conformational assignments regarding the orientation of the phenolic OH group. Cyclic hydrogen-bonding linkage formed in the monohydrated cluster pair is found to be sensitive to the orientation of the phenolic OH group. One of the cluster pair, in which tyramine has the gauche-cis conformation, is more stabilized by the cyclic hydrogen bonding and its origin band intensity becomes stronger than that of the other.

Laser spectroscopic study on the conformations and the hydrated structures of benzo-18-crown-6-ether and dibenzo-18-crown-6-ether in supersonic jets

The laser-induced fluorescence spectra of jet-cooled benzo-18-crown-6 (B18C6) and dibenzo-18-crown-6 (DB18C6) exhibit a number of vibronic bands in the 35 000-37 000 cm(-1) region. We attribute these bands to monomers and hydrated clusters by fluorescence-detected IR-UV and UV-UV double resonance spectroscopy. We found four and two conformers for bare B18C6 and DB18C6, and the hydration of one water molecule reduces the number of isomers to three and one for B18C6-(H(2)O)(1) and DB18C6-(H(2)O)(1), respectively. The IR-UV spectra of B18C6-(H(2)O)(1) and DB18C6-(H(2)O)(1) suggest that all isomers of the monohydrated clusters have a double proton-donor type (bidentate) hydration. That is, the water molecule is bonded to B18C6 or DB18C6 via two O-H[dot dot dot]O hydrogen bonds. The blue shift of the electronic origin of the monohydrated clusters and the quantum chemical calculation suggest that the water molecule in B18C6-(H(2)O)(1) and DB18C6-(H(2)O)(1) prefers to be bonded to the ether oxygen atoms near the benzene ring.

Evaluation of Hydration Enthalpies of Monatomic Cations by Considering Both Long-Range and Short-Range Interactions

The standard hydration enthalpy, ΔH°hyd, of a monatomic cation was calculated as the sum of (1) the enthalpy due to the long-range interaction between a hydrated ion and bulk water, ΔH°LR, (2) the enthalpy due to the short-range interaction between the ion and water molecules in the first hydration shell, ΔH°SR, and (3) the enthalpy due to the ligand field stabilization of an ion, ΔH°LF, which arises for a transition-metal ion. ΔH°LR was estimated on the basis of the Born theory assuming the radius of the hydrated ion as the interatomic distance between the ion and the oxygen atom of a water molecule in the first hydration shell, rM-O, determined experimentally. ΔH°SR was evaluated on the basis of the donor−acceptor interaction between an ion and a water molecule coordinating to the ion, which was evaluated by the molecular orbital calculation of a monohydrated cluster of an ion combined with the Mulliken population analysis. ΔH°LF was calculated on the basis of the crystal field theory. Hydration enthalpies of 48 monatomic cations thus calculated agreed well with those observed experimentally.

Energetics of monohydrated chiral R(+)-1-phenyl-1-propanol: supersonic beam experiments and density functional calculations

Isolated hydrated clusters of chiral R(+)-1-phenyl-1-propanol have been studied in a supersonic molecular beam by means of one- and two-color resonant two-photon ionization mass-resolved spectroscopy. Excitation and threshold ionization spectra have been determined for monohydrated clusters together with the electronic ground state binding energy. Cluster spectra have been analyzed with the aid of ab initio molecular orbital calculations conducted at the B3LYP/6-31G ∗∗ level of the theory. The analysis gives information on the cluster structures and the binding energies.

On the deprotonation of the cation radicals of substituted ethylenes. A heterolytic or homolytic process?

Abstract The deprotonation of several cation radicals of substituted ethylenes in a monohydrated, monoammoniated or dihydrated cluster has been theoretically studied using an ab initio method through the 6-31G(D, P) basis set. It has been found that the cation radicals are π radicals, whereas the corresponding deprotonated species are σ radicals, the deprotonation process involving an intramolecular electron transfer from a σ to a π molecular orbitals. The experimental findings about the deprotonation reactions of organic cation radicals that have charge delocalized from the departing proton have been confirmed. The dissociation of the carbon-hydrogen bond begins with the homolytic stretching of the CH bond and then becomes a heterolytic dissociation, the transition state appearing very near the region of the crossing between both diabatic potential energy surfaces. The global proton transfer can therefore be described as a concerted transfer of a hydrogen atom and an electron in opposite senses.