Effect Of Methyl Substitution Research Articles

Effects of Methyl Substitution on the Ultrafast Internal Conversion of Benzene.

The effects of methyl substitution on the ultrafast internal conversion from the S2(1B1u, ππ*) state to the S0 state of benzene were studied using ultrafast extreme-ultraviolet photoelectron spectroscopy and electronic structure calculations. The quantum yield of the internal conversion to the S0 state reached ∼0.69 in benzene, while lower values of 0.35 and 0.12 were obtained for toluene and o-xylene, respectively. These results indicate that methyl substitution makes the conical intersections less accessible to the nuclear wave packet.

Insights into Ultrafast Relaxation Dynamics of Electronically Excited Furfural and 5-Methylfurfural.

The ultrafast relaxation dynamics of furfural and 5-methylfurfural following excitation in the ultraviolet range is investigated using the femtosecond time-resolved photoelectron spectroscopy method. Specifically, the pump wavelength-dependent decay dynamics of electronically excited furfural and 5-methylfurfural is discussed on the basis of a detailed analysis of our measured time-resolved photoelectron spectroscopy spectra. Irradiation at all pump wavelengths prepares both furfural and 5-methylfurfural molecules with different vibrational levels in the first optically bright S2 (1ππ*) state, the lifetime of which is measured to be at least hundreds of femtoseconds. Besides the prominent deactivation channels of ring-opening and ring-puckering pathways for the S2(1ππ*) state, we propose that there is a minor decay channel of internal conversion from the initially prepared S2(1ππ*) state to the S1(1nπ*) state. The wavepacket decays out of the Franck-Condon region on the S2(1ππ*) state potential energy surface and bifurcates into different parts somewhere. A small fraction of the wavepacket funnels down to the S1(1nπ*) state via internal conversion. The subsequently populated S1(1nπ*) state contains large vibrational excess energy and decays over a lifetime of 2.5-2.8 ps. One of the deactivation channels of the S1(1nπ*) state is intersystem crossing to the 3ππ* triplet state. In addition, methyl substitution effects on the excited-state dynamics of furfural are also discussed. This experimental study provides new insights into the excitation energy-dependent decay dynamics of photoexcited furfural and 5-methylfurfural.

Effect of methyl substitution on hydrogen bond structure of anthocyanin

AbstractIn nature, hydrogen bonding is a common physical occurrence that has a significant impact on the surroundings of anthocyanins. Water molecules will create hydrogen bonds with anthocyanin molecules in various configurations, but the characteristics of these hydrogen bonds will change. Varied hydrogen bonding characteristics have varied impacts on solvent solutions. This research analyzes the differences in hydrogen bonding qualities caused by different methyl structures, as well as the underlying explanations. In this study, the cyanidin and peonidin structures of anthocyanin molecules were calculated in various stable hydrogen bond configurations using the density functional theory B3LYP/6‐31G(d,p), combined with information from the infrared spectroscopy spectrum, atoms in molecules analysis, interaction energy E, and intermolecular hydrogen bond length. The hydrogen bond structure that is the most stable is determined by analyzing it, as well as the effects of replacing the hydroxyl group with a methyl group and any potential underlying causes. Future anthocyanins research can benefit from the precise theoretical reference that this study can offer.

Excitation Energy-Dependent Decay Dynamics of the S1 State of N-Methyl-2-pyridone.

The UV-induced decay dynamics of N-methyl-2-pyridone is investigated using a femtosecond time-resolved photoelectron spectroscopy method. Irradiation in the wavelength range of 339.3-258.9 nm prepares N-methyl-2-pyridone molecules with very different vibrational levels of the S1(11ππ*) state. For v' = 0 (origin) and a few low-energy vibrational levels slightly above the S1 state origin, the radiative decay channel is in operation for some specific vibrations. This is revealed by the excited-state lifetime of ≫1 ns. In addition, some other nearby S1 vibronic states have a much shorter lifetime in the range of several picoseconds to a few tens of picoseconds, indicating that the radiation-less decay to the ground state (S0) via internal conversion is the dominant channel for them. As the pump wavelength slightly decreases, the radiative decay is suddenly not important at all, and the deactivation rate of the S1 state becomes faster. At shorter pump wavelengths, the lifetime of highly excited vibrational states of the S1 state further decreases with the increase in the vibrational excess energy. This study provides quantitative information about the excitation energy-dependent decay dynamics of the S1 state of N-methyl-2-pyridone. Methyl substitution effects on the excited-state dynamics of 2-pyridone are also discussed.

Assessment of the Electron Correlation Treatment on the Quantum-Classical Dynamics of Retinal Protonated Schiff Base Models: XMS-CASPT2, RMS-CASPT2, and REKS Methods.

We compare the performance of three different multiconfigurational wave function-based electronic structure methods and two implementations of the spin-restricted ensemble-referenced Kohn-Sham (REKS) method. The study is characterized by three features: (i) it uses a small set of quantum-classical trajectories rather than potential energy surface mapping, (ii) it focuses, exclusively, on the photoisomerization of retinal protonated Schiff base models, and (iii) it probes the effect of both methyl substitution and the increase in length of the conjugate π-system. For each tested method, the corresponding analytical gradients are used to drive the quantum-classical (Tully's FSSH method) trajectory propagation, including the recent multistate XMS-CASPT2 and RMS-CASPT2 gradients. It is shown that while CASSCF, XMS-CASPT2, and RMS-CASPT2 yield consistent photoisomerization dynamics descriptions, REKS produces, in some of these systems, qualitatively different behavior that is attributed to a flatter and topographically different excited state potential energy surface. The origin of this behavior can be traced back to the effect of the employed density functional approximation. The above studies are further expanded by benchmarking, at the CASSCF and REKS levels, the electronic structure methods using a QM/MM model of the visual pigment rhodopsin.

Effects of Methyl Substitution and Leaving Group on E2/SN2 Competition for Reactions of F- with RY (R = CH3, C2H5, iC3H7, tC4H9; Y = Cl, I).

The competition between base-induced elimination (E2) and bimolecular nucleophilic substitution (SN2) is of significant importance in organic chemistry and is influenced by many factors. The electronic structure calculations for the gas-phase reactions of F- + RY (R = CH3, C2H5, iC3H7, tC4H9, and Y = Cl, I) are executed at the MP2 level with aug-cc-pVDZ or ECP/d basis set to investigate the α-methyl substitution effect. The variation in barrier height, reaction enthalpy, and competition of SN2/E2 as a function of methyl-substitution and leaving group ability has been emphasized. And the nature of these rules has been explored. As the degree of methyl substitution on α-carbon increases, the E2 channel becomes more competitive and dominant with R varying from C2H5, iC3H7, to tC4H9. Energy decomposition analysis offers new insights into the competition between E2 and SN2 processes, which suggests that the drop in interaction energy with an increasing degree of substitution cannot compensate for the rapid growth of preparation energy, leading to a rapid increase in the SN2 energy barrier. By altering the leaving group from Cl to I, the barriers of both SN2 and E2 monotonically decrease, and, with the increased number of substituents, they reduce more dramatically, which is attributed to the looser transition state structures with the stronger leaving group ability. Interestingly, ∆E0‡ exhibits a positive linear correlation with reaction enthalpy (∆H) and halogen electronegativity. With the added number of substituents, the differences in ∆E0‡ and ∆H between Y = Cl and I likewise exhibit good linearity.

A structure-property study for the effect of methyl substituents on the gas separation of spirodifluoranthene-based polymers of intrinsic microporosity

A simple methyl substitution on the structural building blocks of a PIM can alter its chain stacking behavior. Hence, we have introduced differing amounts of methyls on bulky and rigid spirodifluoranthene (EN) configured as two fluorenyl groups perpendicular to anthracene to obtain a series of spirodifluoranthene-based PIMs with increased intrinsic microporous, as demonstrated by nitrogen adsorption and enhanced gas permeability. The methyl substitution increased the free volume of polymer membranes, but the free volume does not increase in proportion to the number of methyl groups, and the increment is relatively slight at high methyl substitution amounts. Also, the effect of methyl substitution was closely associated with the molar contents of EN structures in PIMs. The series of polymers exhibited excellent CO2/N2, CO2/CH4, and O2/N2 gas separation performances, ranking among state-of-the-art polymeric materials for gas separation. Particularly, PIM-OMEN-40 exhibited the highest CO2 permeability, up to 12,431 barrer, which corresponds to 2.94 times higher than that of PIM-1 and along with comparable gas selectivity. Even after more than 100 days of aging, the series of PIMs still showed higher gas permeability and selectivity than fresh PIM-1, demonstrating great potential in separation areas for CO2 capture and storage, natural gas and biogas upgrading of prime importance to energy and the environment.

Taming of the DIBAL Promoted Debenzylation of α-Cyclodextrin. Kinetics, Substituent Effects and Efficient Synthesis of Lings Tetrol.

The kinetics of the reaction of perbenzyl α‐cyclodextrin was studied varying the concentration of DIBAL and substrate, and the temperature. The initial debenzylation was found to be 1st order in substrate and follow the relationship 0.0675+0.179[DIBAL]2 with respect to the concentration of DIBAL. The second and the third debenzylation which led to the 3A,6A,6D‐triol (Lings triol) were both found to be 1st order in substrate concentration and zero order in DIBAL concentration. Longer reaction times with DIBAL in high concentration gave further debenzylation to comparatively complex mixtures containing the 2B,3A,6A,6D‐tetrol and the 3A,6A,6C,6D‐tetrol. In contrast reaction at 0.1 M DIBAL gave the symmetrical 3A,6A,3D,6D‐tetrol (Lings tetrol) in 60 % yield. The effect of chlorine or methyl substitution of the phenyl groups of perbenzyl α‐cyclodextrin was also investigated. Per 4‐chlorobenzyl slowed down the reaction with DIBAL, while 4‐methylbenzyl increased the reaction rate, but still gave the corresponding 6 A‐monool or 6A,6D‐diol products. A Hammett reaction constant of −4.9 was found for the first debenzylation showing a high degree of positive charge in the transition state. The per(2,4‐dichlorobenzyl)‐α‐cyclodextrin‐derivative was completely resistant to DIBAL, however upon addition of trimethyl aluminium this derivative also reacted to give the 6A,6D‐diol product.

Excited State Dynamics of Methylated Guanosine Derivatives Revealed by Femtosecond Time-resolved Spectroscopy.

Methylated DNA/RNA nucleobases are important epigenetic marks in living species and play an important role for targeted therapies. Moreover, methylation could bring significant changes to the photo-stability of nucleic acid, leading these sites become mutational hotspots for disease such as skin cancer. While a number of studies have demonstrated the relationship between excited state dynamics and the biological function of methylated cytosine in DNA, investigations aimed at unraveling the excited state dynamics of methylated guanosine in RNA have been largely overlooked. In this work, influence of methylation on the excited state dynamics of guanosine is studied by using femtosecond time-resolved spectroscopy. Our results suggest that the effect of methyl substitution on the photophysical properties of guanosine is position sensitive. N1-methylguanosine shows very similar excited state dynamics as that in guanosine, while almost one order of magnitude longer lifetime of the La state is observed in N2, N2-dimethylguanosine. Notably, N7-methylation can lead to a new minimum on the La state and the excited state lifetime is two orders of magnitude longer than that of guanosine. These findings not only help understanding excited state dynamics of methylated guanosines, but also lay the foundation for further studying DNA/RNA strands incorporated with these bases.

Two-dimensional self-assemblies of azobenzene derivatives: effects of methyl substitution of azobenzene core and alkyl chain length.

Elucidating the correlation between the molecular arrangement and physical properties of organic compounds is critical to facilitating the development of advanced functional materials. X-ray structural analyses are generally performed to clarify this relationship. Several attempts have been made to ascertain the links between three-dimensional (3D) crystals and their two-dimensional (2D) structures, which can be revealed by scanning tunnelling microscopy (STM) at the molecular level. Thus, 2D self-assemblies of a series of azobenzene derivatives were investigated in this study, and the effects of methyl substitution of the azobenzene core and alkyl chain length on the 2D molecular arrangements at the solid/liquid interface were revealed. Three types of azobenzene derivatives were prepared; these contained azobenzene (Az), 3-methyl azobenzene (MAz), or 3,3'-dimethyl azobenzene (DAz) as cores and alkyloxy chains of different lengths (C8-13) at their 4,4' positions. The 2D structures of the Az and DAz compounds were found to be modulated owing to the odd-even effect of the alkyl chains in a specific chain-length range; this effect was only weakly exhibited by the MAz compounds. This result suggests that only the methyl-group substitution of the azobenzene core significantly affected the 2D structures. The 2D structural features have been discussed in terms of molecular conformation, as well as their correlation with the photo-melting behaviour of the azobenzene derivatives, particularly the MAz compounds.

Observation of Conformational Simplification upon N-Methylation on Amino Acid Iodide Clusters.

This Letter reports a counterintuitive observation that methylation of the glycine-iodide cluster leads to fewer conformations and spectroscopic simplicity. Cryogenic "iodide-tagging" negative ion photoelectron spectroscopy (NIPES) is used to probe specific binding sites of three N-methylated glycine derivatives, i.e., N-methylglycine (sarcosine), N,N-dimethylglycine, and N,N,N-trimethylglycine (glycine betaine). NIPES reveals a progressive spectral simplification of the iodide clusters with increasing methylation due to fewer contributing structures. Low energy conformers and tautomers of each cluster are computationally identified, and those observed in the experiments are assigned based on excellent agreement between the NIPE spectra and theoretical simulations. Zwitterionic cluster structures are found to be less stable than their canonical forms and do not contribute to the observed spectra. This work demonstrates the power of iodide-tagging NIPES in probing conformations of amino acid-iodide clusters and provides a molecular level understanding on the effect of methyl substitution on amino acid binding sites.

Effect of side-group methylation on the performance of methacrylamides and methacrylates for dentin hybridization

The stability of the bond between polymeric adhesives to mineralized substrates is crucial in many biomedical applications. The objective of this study was to determine the effect of methyl substitution at the α- and β-carbons on the kinetics of polymerization, monomer hydrolytic stability, and long-term bond strength to dentin for methacrylamide- and methacrylate-based crosslinked networks for dental adhesive applications. MethodsSecondary methacrylamides (α-CH3 substituted=1-methyl HEMAM, β-CH3 substituted=2-methyl HEMAM, and unsubstituted=HEMAM) and OH-terminated methacrylates (α- and β-CH3 mixture=1-methyl HEMA and 2-methyl HEMA, and unsubstituted=HEMA) were copolymerized with urethane dimethacrylate. The kinetics of photopolymerization were followed in real-time using near-IR spectroscopy. Monomer hydrolysis kinetics were followed by NMR spectroscopy in water at pH 1 over 30 days. Solvated adhesives (40 vol% ethanol) were used to bond composite to dentin and microtensile bond strength (μTBS) measured after 24h and 6 months storage in water at 37°C. ResultsThe rate of polymerization increased in the following order: OH-terminated methacrylates≥methacrylamides>NH2-terminated methacrylates, with minimal effect of the substitution. Final conversion ranged between 79% for 1-methyl AEMA and 94% for HEMA. 1-methyl-HEMAM showed the highest and most stable μTBS, while HEMA showed a 37% reduction after six months All groups showed measurable degradation after up to 4 days in pH 1, with the methacrylamides showing less degradation than the methacrylates. Additionally, transesterification products were observed in the methacrylamide groups. SignificanceAmide monomers were significantly more stable to hydrolysis than the analogous methacrylates. The addition of a α- or β-CH3 groups increased the rate of hydrolysis, with the magnitude of the effect tracking with the expected base-catalyzed hydrolysis of esters or amides, but opposite in influence. The α-CH3 substituted secondary methacrylamide, 1-methyl HEMAM, showed the most stable adhesive interface. A side reaction was observed with transesterification of the monomers studied under ambient conditions, which was not expected under the relatively mild conditions used here, which warrants further investigation.

Design strategy for blue thermally activated delayed fluorescence: Position and methyl substitutions

Wise design strategies for efficient blue TADF molecules are highly desired. In this work, the position effect and methyl substitution effect on the photophysical properties of nine blue TADF molecules are studied. Results show that the introduction of methyl in acceptor unit brings remarkable blue shit for emission. Meanwhile, substitution by methyl in donor unit can bring remarkable TADF properties by facilitating the reverse intersystem crossing and radiative decay processes. Thus, the TADF properties can be improved by the substitution of methyl group in donor unit and deep blue luminescence can be achieved through the substitution effect in acceptor unit.

The detailed electronic structure, spectroscopic features, and reactivity of dimethylanisoles

The spectroscopic features, reactive natures, molecular docking, and electronic properties of anisole and six dimethylanisoles (DMAs) are investigated thoroughly to have insight into the effect of methyl substitution in anisole derivatives. The spectroscopic properties of studied molecules, including FT-IR, FT-Raman, and 1H and 13C NMR spectra were have been obtained by using DFT calculations at B3LYP/6–311++G(d,p) level of theory, and compared with the ones measured experimentally. Reactivity features were determined based on global descriptors, electron density mapping, and Fukui analysis to reveal the electrophilic and nucleophilic nature of them. The methyl position mainly affects the reactivity of the methoxy group and the aromatic nature of the ring. The methyl position dependency of aromaticity and π-stacking ability in DMAs have been explored through the nuclear independent chemical shielding (NICS) calculations, aromaticity indexes, and π orbital overlapping analysis. The static and dynamic polarizability and first and second hyperpolarizability have been carried out to explore the potential of the molecules in optical applications and the effect of methyl position on these characteristics. Bioactive potency and toxicity characteristics have been tested through molecular docking of the molecules to the odorant-binding protein.

Molecular thermodynamic understanding of transport behavior of CO2 at the ionic liquids‐electrode interface

AbstractThe transport behavior of CO2 at the ionic liquids (ILs)‐electrode interface was revealed from the thermodynamic view via molecular dynamics simulations. The hopping and self‐diffusive mechanisms were identified in the interfacial and bulk region, and thereafter a hopping‐diffusion model was developed to evaluate the transport resistance of CO2 from bulk to the interface. Meanwhile, the vibrational spectrum and entropy change of CO2 at the interface were calculated using the thermodynamic analysis method. For ILs with the same cation ([Emim]+), both transport resistance and entropy decrease follow the order: [BF4]− < [AC]− < [NO3]−, indicating [BF4]− possesses the faster CO2 transport efficiency across the electrical double layer. Furthermore, the methyl substitution effect on transport and thermodynamic properties was clarified, indicating the coupling relation between the transport process and thermodynamic advantage. These findings can lay the ground for the molecule design of ILs‐electrode interface in the applications in the chemical engineering field.

Effect of Methyl-substitution of the Quinoline Ring of Cinchona Alkaloids on the Performance as a Modifier for the Enantioselective Hydrogenation of (E)-2,3-di(4-methoxyphenyl)propenoic Acid Over Pd/C: Kinetic Analysis

The asymmetric hydrogenation of (E)-2,3-di(4-methoxyphenyl)propenoic acid was conducted over Pd/C chirally modified with cinchona alkaloids methyl-substituted at 2′-position of the quinoline ring. It is revealed that the adsorption strength of the modifiers and the intrinsic enantioselectivity at the modified sites are decreased by the methyl-substitution. The intrinsic enantioselectivity of the modifier is correlated to kinetic parameters.

Substituent effects on nonadiabatic excited state dynamics: Inertial, steric, and electronic effects in methylated butadienes.

The photochemical dynamics of double-bond-containing hydrocarbons is exemplified by the smallest alkenes, ethylene and butadiene. Chemical substituents can alter both decay timescales and photoproducts through a combination of inertial effects due to substituent mass, steric effects due to substituent size, and electronic (or potential) effects due to perturbative changes to the electronic potential energy surface. Here, we demonstrate the interplay of different substituent effects on 1,3-butadiene and its methylated derivatives using a combination of ab initio simulation of nonadiabatic dynamics and time-resolved photoelectron spectroscopy. The purely inertial effects of methyl substitution are simulated through the use of mass 15 "heavy-hydrogen" atoms. As expected from both inertial and electronic influences, the excited-state dynamics is dominated by pyramidalization at the unsubstituted carbon sites. Although the electronic effects of methyl group substitution are weak, they alter both decay timescales and branching ratios by influencing the initial path taken by the excited wavepacket following photoexcitation.

Synthesis and thermal properties of phenol- and amine-capped main-chain benzoxazine oligomers with multiple methyl substitutions

A series of main-chain benzoxazine oligomers with different methyl substitutions are successfully synthesized. Chemical structures are analyzed by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and gel permeation chromatography. Effects of methyl substitutions on chemical shifts of protons in oxazine ring and thermal properties, including glass transition temperature, thermal stability, and char yield, are discussed. The influences of methyl substitutions on different positions are demonstrated: (i) substitution on phenols induces obvious increase in curing temperature while substitution on amine does not show apparent impact; (ii) substitution at different positions results in T g variation, following the sequence of none-substitution > substitution at end-capping > substitution on diamines in main-chain > substitution on bisphenols in main-chain; and (iii) substitution at end-capping would cause apparent deterioration in thermal stability while substitution on diamines in main-chain would benefit thermal stability and char yield. Experimental results and related explanations are provided in detail.

Effect of methyl-substitution on carbazole/oxadiazole donor-acceptor (D-A) type host materials for efficient solution-processed green organic light-emitting diodes

Two new bipolar hosts methyl-substituted carbazole/oxadiazole donor-acceptor (D-A) type compounds of 2,5-bis(2-(3,6-dimethyl-9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (3,6-MeCzOXD) and 2,5-bis(2-(2,7-dimethyl-9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (2,7-MeCzOXD) are reported. The thermostability and solubility were greatly improved by attaching two methyl groups on carbazole in contrast to that of parent o-CzOXD without any substituents. The different positions of termination between the methyl substituents and the carbazole unite have almost no impact on thermal stability, whereas methyl-substituted carbazole at 3,6-positions shows more effective π-conjugation and significantly better electrochemical stability than 2,7-positions. The triplet energies of 3,6-MeCzOXD and 2,7-MeCzOXD were measured to be 2.56 and 2.64 eV. Solution-processed green PhOLEDs based on the two new o-CzOXD derivatives as hosts were achieved with CEmax of 32 cd A−1 and PEmax of 26 lm W−1. Besides, green TADF devices employing 3,6-MeCzOXD as host material maintain the values with CEmax of 32.4 cd A−1 and PEmax of 17.2 lm W−1.

On single-electron magnesium bonding formation and the effect of methyl substitution.

The complexes formed between MgX2 (X = F, H) molecules and alkyl radicals Y [Y = CH3, CH2CH3, CH(CH3)2, and C(CH3)3] have been characterized by using quantum chemical methods. The binding distance in all cases is less than the sum of vdW radii of Mg and C, indicating the formation of a non-covalent interaction, namely single-electron magnesium bond. Energy decomposition analysis reveals that electrostatic and polarization contributions are the major components responsible for the stability of the studied complexes. According to interaction energy, atoms in molecules, and independent gradient model analyses, methyl substitution on electron donor Y imposes a positive effect on its complexation with MgX2. When compared with other nonbonded interactions, the single-electron magnesium bond is found to have strength comparable to those of the single-electron beryllium bond and π-magnesium bond.