CH Stretch Bands Research Articles

Experimental Determination of the Unusual CH Stretch Frequency of Protonated Fullerenes

We report experimental values for the CH stretch frequencies of the protonated fullerenes C60H+ and C70H+. Anharmonic frequency calculations at the B3LYP/6-31G level of theory, which are independent of empirical scaling factors, reproduce the experimental values to within approximately 5 cm−1. Scaling theoretical harmonic frequencies by applying factors derived for polycyclic aromatic hydrocarbons deviate significantly from the experimentally measured frequency. We attribute this deviation to the unusual hydrocarbon structure that affects the degree of anharmonicity of the CH stretch. This result allows us to propose an original, specific scaling factor of 0.9524 to correct harmonic frequencies of CH stretches of protonated fullerenes calculated at the B3LYP/6-311 + G(d,p) level of theory. The special spectral position of the protonated fullerene CH stretch bands makes it a diagnostic marker that may aid in their detection in the interstellar medium.

When Phenyl and Cyclopropyl Rings Meet: Spectroscopic Shifts and Conformational Questions.

The electronic and vibrational spectra of cyclopropylbenzene (CPB) and 1,3-bromocyclopropylbenzene (BrCPB) in the gas phase were investigated using quantum chemical calculations in combination with resonance-enhanced multi-photon ionization REMPI techniques including 1c-R2PI, UV-UV holeburning, and IR-UV ion depletion in the CH stretch region. The electronic spectra revealed the presence of a single conformer for both species, with the absence of any perpendicular conformer attributed to low computed barriers to conformer interconversion. Assignment of CPB to the bisected conformer was made through interpreting distinctive CH stretch bands in the IR-UV spectrum in conjunction with quantum chemical calculations. A local anharmonic model based on DFT calculations was adapted to reproduce the cyclopropyl CH stretch spectrum successfully. It was not feasible to definitively assign which bisected conformer of BrCPB was observed using vibrational information alone due to the close similarity of their predicted IR spectra. However, conformational sensitivity of the S1 ← S0 transition dipole moment (TDM) alignments leads to simulated rotational contours that display stark differences, which prompted assignment to the "B1" bisecting conformer with the cyclopropyl ring directed away from the bromine atom. The absence of the energetically comparable "B2" conformer is unexpected. The analysis of the convolution of aromatic and aliphatic modes serves as a basis for assignment in constrained aliphatic systems.

Infrared spectrum of the 1-cyanoadamantane cation: evidence of hydrogen transfer and cage-opening upon ionization.

The radical cations of diamondoids are important intermediates in their functionalization reactions and are also candidates as carriers for astronomical absorption and emission features. Although neutral diamondoids have been studied extensively, information regarding their radical cations is largely lacking, particularly for functionalized diamondoid derivatives. Herein, we characterize the structure of the 1-cyanoadamantane radical cation (C10H15CN+, AdCN+) using infrared photodissociation (IRPD) spectroscopy of mass selected AdCN+N2 clusters in the XH stretch range (2400-3500 cm-1) and dispersion-corrected density functional theory calculations (B3LYP-D3BJ/cc-pVTZ). A group of three distinct CH stretch bands are observed in the 2800-3000 cm-1 range, in addition to a highly redshifted absorption at 2580 cm-1 attributed to the acidic CH proton predicted by calculations. An unexpected broad absorption peaking at 3320 cm-1 is also detected and assigned to an NH stretch mode based on its width and frequency. Calculations indicate that hydrogen atom transfer (HAT) from the adamantyl cage (C10H15, Ady) to the N atom of the CN group yields lower energy structures, with an open-cage isomer exhibiting such hydrogen transfer being the global minimum on the potential energy surface. The energy barriers involved in the formation of this open-cage isomer are also lower than those calculated for generation of the analogous open-cage 1-amantadine cation isomer which has previously been identified by IRPD. The combined consideration of IRPD spectra and calculations indicates a major population of the nascent canonical closed-cage isomer and a smaller population of the global minimum isomer featuring both cage-opening and hydrogen transfer.

Gas-phase CH-Overtone band spectra of methyl acetate and ethyl acetate via incoherent broad-band cavity-enhanced absorption spectroscopy

Gas-phase overtone bands of methyl acetate and ethyl acetate were detected for up to v = 6–0 by incoherent broad-band cavity-enhanced absorption spectroscopy. The signal-to-noise ratios of v = 4–0 were achieved to be 40–50 by the 100 s integration time, suggesting an application possibility of this spectroscopic technique for environmental monitoring. Profiles of the observed overtone bands were analyzed in detail with the aid of theoretical calculations and their remarkable peaks were assigned to the progressions starting from the symmetric CH-stretching bands. The harmonic frequencies, the anharmonicities, and the dissociation energies were derived.

Study of morphological, structural, and thermal changes of biodegradable material in simulated natural environmental conditions

AbstractPolymers with low environmental impact are of great interest worldwide to reduce greenhouse gases (CO2 and CH4) and soil and water contamination. Evaluating the biodegradation process of biopolymeric matrices obtained from different biodegradable biopolymers is a priority. This work evaluated the behavior of a foamed material based on biopolymers under the ISO 14855‐2: 2007 standard under simulated natural environmental conditions and its ecotoxic potential of the material residues in the growth of seeds Phaseolus vulgaris. A percentage of biodegradation was obtained in the TPS/PLA samples: 84.7 ± 0.3%; 0% MG: 89.1 ± 0.4%; 2% MG: 82.4 ± 0.4%; 3% MG: 82.0 ± 0.%1; 6% MG: 81.0 ± 0.3%; TPSEG: 66.7 ± 2.4%, and TPSE: 62.9 ± 1.0%; having microcrystalline cellulose as a control sample. CH stretch bands were identified at 728 cm–1 related to PBAT vibration that remained constant in the samples containing PHA, evidencing that this material is not an alternative to elaborate foamed materials to improve their biodegradation. Protein‐based samples increase their biodegradation rate, while based samples PHA remain resilient over time due to the blend of the polymer and the presence of PLA (1437.5 cm–1). The low or high decomposition of the material in weight loss corroborated this behavior of both blends. Regarding the ecotoxicity of the samples, there was no significant difference.

High-resolution infrared spectroscopy of supersonically cooled singlet carbenes: Bromomethylene (HCBr) in the CH stretch region.

First high-resolution spectra of cold (∼35K) singlet bromomethylene HCBr in the CH stretching (v1) region from 2770 to 2850cm-1 are reported using near quantum shot-noise limited laser absorption methods in a slit jet supersonic discharge expansion source. Three rovibrational bands are identified at high S/N (20:1-40:1) and rotationally assigned to (i) the CH stretch fundamental (v1) band X̃1,0,0←X̃0,0,0 and (ii) vibrational hot bands [X̃(1,1,0)←X̃(0,1,0) and X̃(1,0,1)←X̃(0,0,1)] arising from vibrationally excited HCBr populated in the discharge with single quanta in either the H-C-Br bend (v2) or C-Br stretch (v3) modes. Precision rotational constants are reported for a total of six states, with an experimentally determined CH stretch vibrational frequency (2799.38cm-1) in good agreement with previous low-resolution fluorescence studies [M. Deselnicu et al., J. Chem. Phys. 124(13), 134302 (2006)]. Detailed analysis of the fundamental v1 band highlights the presence of perturbations in the X̃1,0,0 level, which we tentatively attribute to arise from the nearby triplet state ã(0,0,1) through spin-orbit interaction or the multiple quanta X̃0,2,1 singlet state via c-type Coriolis coupling. Reduced-Doppler resolution (60MHz) in the slit-jet IR spectrometer permits for clear observation of a nuclear spin hyperfine structure, with experimental line shapes well reproduced by nuclear quadrupole/spin-rotation coupling constants from microwave studies [C. Duan et al., J. Mol. Spectrosc. 220(1), 113-121 (2003)]. Finally, the a-type to b-type transition intensity ratio for the fundamental CH stretch band is notably larger than that predicted by using a bond-dipole model, which from high level ab initio quantum calculations [CCSD(T)/PVQZ] can be attributed to vibrationally induced "charge-sloshing" of electron density along the polar C-Br bond.

Label-free spectroscopic characterization of exosomes reveals cancer cell differentiation

Exosomes (EXOs) are considered an exceptionally promising source of cancer biomarkers for personalized medicine and liquid biopsy. Despite this potential, the EXOs translation process in diagnostics is still at its birth, and the development of reliable and reproducible methods for their characterization is highly demanded. Fourier Transform Infrared (FTIR) Spectroscopy is perfectly suited for this purpose, as it can provide a label-free biochemical profile of EXOs in terms of lipid, protein, and nucleic acid content. Here we evaluated the applicability of FTIR spectroscopy to the study of cancer-derived EXOs as a function of cell differentiation. For this purpose, we used N-acetyl-l-Cysteine (NAC) to induce a controlled differentiation in human colon carcinoma cells from a proliferative mesenchymal morphology to a less invasive epithelial phenotype, as measured with fluorescence and electron microscopy. EXOs derived from cells with different phenotypes showed significant variation in the relative intensity of the amide I-II and CH-stretching bands in the mid-IR range, indicating the spectroscopic lipid/protein ratio as an effective classification parameter. Additionally, we showed that different cell phenotypes are associated with a shape modification in these spectral bands that can be automatically detected by combining Principal Component Analysis (PCA) with Linear Discriminant Analysis (LDA).On the one hand, our study confirms that an FTIR analysis of EXOs allows scientists to precisely detect modifications occurring at the parental cell level; on the other hand, it unveils a set of effective spectral biomarkers able to monitoring cell changes from a mesenchymal to an epithelial phenotype, a clinically valuable piece of information considering that the epithelial-to-mesenchymal transition is a key step in the metastatic process.

Cooperativity of the Activated CH/π Interaction Probed through CH Stretching Vibrations in Phenol-(Acetylene)n (∼16 ≤ n ≤ ∼30) and (Acetylene)n+ (10 ≤ n ≤ 70) Clusters.

The acidity of acetylene CH is stronger than that of alkane CH, and the attractive interaction between an acetylene CH with π-electrons, which shows a clear hydrogen bond property, is called activated CH/π interaction. In this study, cooperative enhancement of the activated CH/π interaction has been probed through the cluster size dependence of the red shift of the acetylene CH stretching vibrational band in neutral phenol-(acetylene)n (∼16 ≤ n ≤ ∼30) and (acetylene)n+ (10 ≤ n ≤ 70). In both the clusters, the characteristic asymmetric (red-shaded) shape of the CH stretch band has been observed. This band shape means that the magnitude of the activated CH/π interaction is enhanced by its cooperativity in the interior moiety of the cluster. The red-shifted component of the band extends with increasing cluster size, and the edge of this component seems to reach to the CH stretch band position of crystalline acetylene at the size of n = 20-30, indicating that dozens of molecules need to interact each other to maximize cooperativity in the activated CH/π interaction of acetylene. On the other hand, the peak position of the band does not converge to that of crystalline acetylene in the observed size range. The present result suggests that the spectral convergence of acetylene clusters to the bulk may occur in the cluster size range of hundreds or larger.

Non long-range corrected density functionals incorrectly describe the intensity of the CH stretching band in polycyclic aromatic hydrocarbons.

We present a comprehensive study of the most relevant numerical aspects influencing frequencies and intensities in the infrared spectrum of isolated polycyclic aromatic hydrocarbons (PAHs) regarding the overestimate of the IR CH-stretching bands. We use naphthalene as benchmark and show the validity of our results to different members of the PAH family. Our analysis relies on widely employed density functional theory methods and second-order vibrational perturbational theory for the computation of vibrational eigenstates. We have focused on the elucidation of the origin of the systematic overestimate of the intensities in the CH-stretching region. To rule out nonfundamental numerical errors, we have initially considered the influence of the electronic basis set and various other parameters on the different stages of the vibrational analysis. In a second stage, we have benchmarked the results of different density functional theory functionals with respect to the aforementioned overestimate taken as the ratio between the most prominent features of the spectrum, the CH-bending and the CH-stretching bands. Our results unambiguously indicate that the long-range correction plays a major role in this spurious numerical issue. More specifically, this phenomenon is due to an incorrect description of the charge distribution (and hence dipole) within the symmetrically relevant CH bonds. Long-range correction specifically remedies this issue. It improves the description of the intensities in the stretching region while at the same time it does not perturb significantly the rest of the spectrum. With respect to the frequencies, we have observed an overall improvement when compared to noncorrected functionals.

Computational Study of the Rovibrational Spectra of CH2D+ and CHD2.

In this paper, we present rovibrational energy levels of CH2D+ and CHD2+. They are computed with a large basis and the Lanczos algorithm. CH2D+ and CHD2+ are believed to play an important role in interstellar space, but so far, there are no definitive observations. The predictions of this paper should facilitate detection. For CH2D+, two CH stretch bands have been studied at high resolution. Compared to our calculated energies, the root-mean-square error is 0.08 cm-1. For CHD2+, one CH stretch band has been studied at high resolution. Compared to our calculated energies, the root-mean-square error is 0.5 cm-1. Errors are larger, for both isotopologues, for bend states. We attribute these errors to the potential energy surface. Wave function and probability distribution plots are used to make assignments. The ν1 band of CHD2+ is significantly perturbed, and according to our calculations, the 3ν3 state is closest and might be the most important perturber.

Infrared Spectroscopic Study on Trimethyl Amine Radical Cation: Correlation between Proton-Donating Ability and Structural Deformation.

Barrierless intermolecular proton transfer from a CH bond has recently been reported in the vertical ionization of the trimethyl amine (TMA) dimer. This result indicates the remarkable enhancement of the proton-donating ability of the CH bond in its cationic state. In the present study, we have carried out an infrared spectroscopy of the neutral and cationic TMA in the CH stretch region and their theoretical calculations to investigate the mechanism of enhancement of the proton-donating ability in the cationic state. In the spectrum of the cation, the CH stretch band shows a long tail of up to 2600 cm-1. This tail component is attributed to the CH bond hyperconjugated with the nonbonding orbital at the nitrogen atom through geometry deformation (excitation of molecular vibrations) with the excess energy upon photoionization. This hyperconjugation causes the delocalization of the σ electron of the CH bond to the singly occupied nonbonding orbital so that the proton-donating ability of the CH is enhanced. It is shown that the excitation of the CN stretching vibration is especially effective in promoting the hyperconjugation.

The S∴π hemibond and its competition with the S∴S hemibond in the simplest model system: infrared spectroscopy of the [benzene-(H2S) n ]+ (n = 1-4) radical cation clusters.

The S∴π hemibond (two-center three-electron, 2c-3e, bond) is an attractive interaction between a sulfur atom and π electrons. The S∴π hemibond is of essential importance in understanding chemistry of sulfur radical cations, and its roles in biochemistry have recently attracted much interest. In the present study, we observe the S∴π hemibond in the simplest model system in the gas phase. Infrared spectroscopy is applied to the [benzene-(H2S) n ]+ (n = 1-4) radical cation clusters. In n = 1, the CH stretch and SH stretch bands of the benzene and H2S moieties, respectively, are clearly different from those of the neutral molecules but similar to those of the ionic species. These vibrational features show that the positive charge is delocalized over the cluster due to the S∴π hemibond formation. In n = 2-4, the S∴S hemibond and S-π-S multicenter hemibond (three-center five-electron, 3c-5e, bond) can compete with the S∴π hemibond. The observed vibrational features clearly indicate that the S∴S hemibond formation is superior to the S∴π hemibond and S-π-S multicenter hemibond. Calculations of several dispersion-corrected density functionals are compared with the observations. While all the tested functionals qualitatively catch the feature of the S∴π hemibond, the energy order among the isomers of the different hemibond motifs strongly depends on the functionals. These results demonstrate that the [benzene-(H2S) n ]+ clusters can be a benchmark of density functionals to evaluate the sulfur hemibonds.

Properties of hydrogen bonds in the protic ionic liquid ethylammonium nitrate

Comparative molecular dynamics simulations of a hexamer cluster of the protic ionic liquid ethylammonium nitrate are performed using density functional theory and density functional-based tight-binding (DFTB) methods. The focus is on assessing the performance of the DFTB approach to describe the dynamics and infrared spectroscopic signatures of hydrogen bonding between the ions. Average geometries and geometric correlations are found to be rather similar. The same holds true for the far-infrared spectral region. Differences are more pronounced for the NH- and CH-stretching bands, where DFTB predicts a broader intensity distribution. DFTB completely fails to describe the fingerprint range shaped by nitrate anion vibrations. Finally, charge fluctuations within the H bonds are characterized yielding moderate dependencies on geometry. On the basis of these results, DFTB is recommended for the simulation of H bond properties of this type of ionic liquid.

Doppler-limited high-resolution spectrum and VPT2 assisted assignment of the C-H stretch of CH2Br2

The Doppler limited non-saturated rotationally resolved infrared spectra of the symmetric and asymmetric CH-stretch bands of CH2Br2 have been measured. A continuous wave cavity ringdown setup with a widely tunable Mid-IR-OPO laser light source yielded a single-shot minimum absorption of 4.9×10−8cm−1. In contrast to the heavily congested ν1 band, the ν6 band showed partially resolved rotational features that may serve as suitable absorption targets in future environmental detection schemes for CH2Br2. A straightforward, VPT2 (second-order vibrational perturbation theory) assisted quantum-chemical approach for assigning the rotational structure has been tested using different model chemistries. The molecular structures, anharmonic frequencies and the structural changes upon vibrational excitation of CH2Br2 have been investigated. The predicted changes of the anharmonic rotational constants have been used together with available spectroscopic ground state constants to simulate the rovibrational structures of the ν1 and ν6 bands of CH2Br2. A refined analysis of the ν6 band is presented yielding accurate values for the band origin and the rotational constants. A fit of the line positions of 312 prominent transitions of the three isotopologues revealed a low standard error of 0.00056cm−1, hence within the absolute 0.0009cm−1 wavelength accuracy of the used spectrometer setup. A combined analysis of the predicted line strengths and positions of the strong Q sub-branches of the ν6 band has been performed to test the ability of the different density functionals for VPT2 prediction of anharmonic molecular constants. The M06/6-311++G(d,p) model chemistry turned out to yield reliable state-dependent rotational constants that are accurate enough to reproduce the overall rotational structure even without fitting.

High-speed PIV, spray, combustion luminosity, and infrared fuel-vapor imaging for probing tumble-flow-induced asymmetry of gasoline distribution in a spray-guided stratified-charge DISI engine

In this study, the influence of intake-generated swirl and tumble flow on fuel–air mixing and combustion is investigated in a spray-guided stratified-charge direct-injection spark-ignited engine. Previously, it was demonstrated that the introduction of a combined swirl–tumble flow recovered combustion stability, which was otherwise lost when increasing the engine speed from 1000 to 2000rpm. However, the improved combustion came at the expense of elevated engine-out soot emissions. Here, high-speed combustion luminosity and PIV measurements at 2000rpm confirm that soot incandescence is more prevalent with high swirl and tumble. The application of high-speed infrared (IR) gasoline-vapor imaging introduced here provides unique insights, revealing that operation with a combination of swirl and tumble generates an asymmetric fuel distribution that spatially correlates with highly luminous sooting combustion. The IR fuel-vapor imaging technique collects line-of-sight mid-infrared thermal emission from the CH stretch band of the heated fuel near a wavelength of 3.4µm. The IR images resolve the penetrating vapor plumes distinctly, demonstrating that the 3.4µm band is suitable for quantitative measurements of vapor penetration during injection. After injection, the IR images provide a qualitative description of fuel-vapor spread without combustion. It is found that the no-swirl case has a symmetric fuel-vapor development during the latter part of the compression stroke. In contrast, for operation with strong swirl and tumble, vapor rotation and the development of an asymmetric and non-uniform fuel-vapor distribution is observed. PIV measurements reveal that the swirl flow dominates the vapor rotation, while the tumble flow appears to be a major reason for the asymmetric fuel-vapor distribution.

Vibrational Sum-Frequency Generation Study of Morpholine at Air–Liquid and Air–Solution Interfaces

The structure and orientation of interfacial morpholine molecules have been investigated, using vibrational sum-frequency generation (VSFG) spectroscopy—a nonlinear surface specific technique. The VSFG spectra with SSP and PPP polarizations have been measured in the CH (2800–3000 cm–1) and the OH (3000–3750 cm–1) stretch regions at the air–morpholine and air–solution interfaces. The vibrational frequencies in the CH stretch region could be observed in VSFG spectra, implying presence of morpholine molecules at the interfaces with net polar orientation. The intensities of the CH stretch bands get enhanced at the air–solution interface of morpholine solution in millimolar concentration, in comparison to that at the air–morpholine interface which is attributed to increase in polar orientation of interfacial morpholine molecules induced by water molecules. In pure morpholine, the most predominant conformation of molecules is equatorial chair, both in the bulk and at the air–morpholine interface. But in aqueous solution of morpholine, the contribution from axial chair conformer is known to increase. This effect, and also a probable change from the chair to the boat/twist boat conformation at the air–solution interface, may contribute to the enhanced intensity of VSFG peaks of CH stretch bands in solution. The VSFG intensities of the OH stretching frequencies of interfacial water molecules are also enhanced in the presence of morpholine, suggesting an increase in net polar orientation of water molecules induced by morpholine molecules. The VSFG spectra were also measured in the presence of 300 mM HCl, which showed indications of protonation of the interfacial morpholine molecules. Addition of HCl to aqueous solution of morpholine alters the orientation of interfacial water molecules significantly, and the enhanced VSFG intensities in the OH region induced by morpholine molecules are almost completely obliterated. The result suggests that orientation of interfacial water molecules in the presence of HCl gets random. However, the effects of HCl in the CH region of the VSFG spectra differ for different stretching bands. In presence of readily ionizable HCl molecules, a large number of ions are generated, which are probably responsible for changing the surface orientation of both water and morpholine molecules.

Infrared Spectroscopic Investigation of the Acidic CH Bonds in Cationic n-Alkanes: Pentane, Hexane, and Heptane.

Radical cations of n-alkanes (pentane, hexane, and heptane) in the gas phase are investigated by infrared predissociation spectroscopy with the argon or nitrogen tagging. All-trans and gauche-involving conformers are identified for these cations by comparisons of observed infrared spectra and vibrational simulations. Intense CH stretch bands are observed in the frequency region lower than the normal alkyl CH stretch frequency. These low frequencies and high intensities of the CH stretch bands are caused by the CH bond weakening and the enhanced positive charge of the hydrogen atoms through the delocalization of the σ electron in the CH bonds. These effects of the delocalization of the σ electron result in the enhanced acidity of the CH bonds. The conformation as well as alkyl chain length dependence of the acidity of the CH bonds is demonstrated by the CH stretch frequency shift trend.

High resolution spectroscopy of jet cooled phenyl radical: The ν1 and ν2 a1 symmetry C-H stretching modes.

A series of CH stretch modes in phenyl radical (C6H5) has been investigated via high resolution infrared spectroscopy at sub-Doppler resolution (∼60 MHz) in a supersonic discharge slit jet expansion. Two fundamental vibrations of a1 symmetry, ν1 and ν2, are observed and rotationally analyzed for the first time, corresponding to in-phase and out-of-phase symmetric CH stretch excitation at the ortho/meta/para and ortho/para C atoms with respect to the radical center. The ν1 and ν2 band origins are determined to be 3073.968 50(8) cm(-1) and 3062.264 80(7) cm(-1), respectively, which both agree within 5 cm(-1) with theoretical anharmonic scaling predictions based on density functional B3LYP/6-311g++(3df,3dp) calculations. Integrated band strengths for each of the CH stretch bands are analyzed, with the relative intensities agreeing remarkably well with theoretical predictions. Frequency comparison with previous low resolution Ar-matrix spectroscopy [A. V. Friderichsen et al., J. Am. Chem. Soc. 123, 1977 (2001)] reveals a nearly uniform Δν ≈ + 10-12 cm(-1) blue shift between gas phase and Ar matrix values for ν1 and ν2. This differs substantially from the much smaller red shift (Δν ≈ - 1 cm(-1)) reported for the ν19 mode, and suggests a simple physical model in terms of vibrational mode symmetry and crowding due to the matrix environment. Finally, the infrared phenyl spectra are well described by a simple asymmetric rigid rotor Hamiltonian and show no evidence for spectral congestion due to intramolecular vibrational coupling, which bodes well for high resolution studies of other ring radicals and polycyclic aromatic hydrocarbons. In summary, the combination of slit jet discharge methods with high resolution infrared lasers enables spectroscopic investigation of even highly reactive combustion and interstellar radical intermediates under gas phase, jet-cooled (Trot ≈ 11 K) conditions.

The large variation in acidity of diethyl ether cation induced by internal rotation about a single covalent bond.

In the IR spectrum of the diethyl ether cation, an extraordinarily intense band, with an extremely broad bandwidth, was observed at 2700 cm(-1), much lower frequency than normal CH stretch frequencies. This band is assigned to the stretch band of the CH bond, which is hyperconjugated with the singly occupied molecular orbital of the oxygen atom. The hyperconjugation causes the delocalization of the σ electron of the CH bond so that it enhances the acidity of the CH bond as well as the CH stretch band intensity. Theoretical simulation shows that the strength of hyperconjugation varies greatly with internal rotation of the ethyl group, and this is reflected in the large width of the observed CH stretch band. These results indicate that the DEE cation drastically changes its property from aprotic to highly acidic by the rotational isomerization of the ethyl group.

On the Al + HCN reaction in helium nanodroplets

The sequential addition of HCN and Al to a helium nanodroplet leads barrierlessly to a bent, planar, 2A′, HCNAl species, which is computed to be 20.3kcal/mol below the separated reactants. A ro-vibrational band near 2690cm−1 is assigned to the ν1 (a′) CH stretch. A Natural Bond Orbital analysis of this reaction product reveals that both ligand-to-metal σ-donation and metal-to-ligand π-donation are responsible for the strong bonding between HCN and Al. A significant donor–acceptor interaction between the C–N and C–H anti-bonding orbitals is responsible for the bent geometry and the 619cm−1 red shift of the CH stretch band upon complexation.