Proceedings of the Indian Academy of Sciences - Chemical Sciences

A normal co-ordinate treatment based on general quadratic forcefield has been applied to N-methyl acetamide molecule. In these calculations, the structure parameters of N-methyl acetamide, as given recently by Katzet al., and the Raman and infra-red data obtained by the authors are used and the potential constants have been obtained. The potential energy distribution among the various symmetry co-ordinates of each normal vibration and the mean amplitudes of vibrations have been calculated.

The dipole moments of a number of substituted benzaldehydes are measured in benzene solution. The angle which the dipole axis of the CHO group makes with the axis of rotation of the group is determined. The observed moments of the ortho-substituted benzaldehydes are compared with the moments calculated for free rotation as well as fors-trans ands-cis orientations of the -CHO group.o-Fluorobenzaldehyde exists mostly in thes-trans conformation.o-Chloro-,o-bromo-ando-nitro-benzaldehydcs also exist in thes-trans conformation; their observed dipole moments are even lower than the values calculated fors-trans forms, indicating mutual induction of the ortho substituents. Though 2,5-dichlorobenzaldehyde is expected to have the same dipole moment as benzaldehyde, the observed moment is significantly lower due to mutual induction of the ortho substituents. 2,5-Dimethylbcnzaldehyde has, however, almost the same moment as benzaldehyde. The dipole moment ofo-methoxybcnzaldchyde is considerably higher than the values calculated for boths-cis ands-trans conformations. An explanation is given for this.o-Hydroxybenzaldehyde exists exclusively in thes-cis form due to internal H-bonding.

An energy cassette, N-butyl-6-(3-phenylethynyl-1,3,7,9-tetramethylBODIPY)-naphthalimide (BON), was constructed with an energy donor and acceptor incorporated in a single molecular system. The energy donor (naphthalimide, NA) and acceptor (BODIPY) were bonded through phenylacetylene, a conjugate linker. Highly twisted molecular conformation was formed due to intramolecular repulsion, forcing the two fluorophores to act as independent chromophores. Therefore, the absorption spectra of BON in a common organic solvent are of superimposition of NA and BODIPY. Upon excitation of UV light (365 nm), a typical BODIPY emission character was observed, indicating an efficient energy transfer from NA moiety to the BODIPY scaffold with a transfer efficiency of 98%. Additionally, the pseudo-Stokes’ shift expanded to around 140 nm, which is largely longer than traditional BODIPY dyes (~ 10 nm). The frontier molecular orbitals analysis shows that there exists a pronounced electron density shifting from HOMO to LUMO, suggesting the efficient energy transfer in BON. A through-space energy transfer cassette was established with the twisted molecular conformation-induced orbital decoupling and favorable mutual orientation of the excited NA/BODIPY moment vector. A dye pair (BON) was configured by naphthalimide and BOIDPY with a conjugate linker, phenylacetylene. Due to intramolecular repulsion in BON, the dye molecule is highly twisted, forming an efficient energy transfer cassette. An efficient energy transfer from NA to BODIPY occurred with a transfer efficiency of 98%.

The constitution of hibiscitrin proposed in Part I as the 3-glucoside of hibiscetin is confirmed here by synthetic experiments. The O-hexamethyl hibiscetin (A) has been ethylated to (B). Using the method of nuclear oxidation a hexamethyl hibiscetin with a free hydroxyl in position 5 is now prepared and shown to be different from (A). Ethyl ethers of the isomeric 8- and 3-hydroxy compounds are also prepared. The former is different from (B) whereas the latter is identical with it. Though alkali fission of (A) does not yield the ketonic part, the ethyl ether (B) can be satisfactorily employed for this fission. The ketonic product is found to be identical with ω-ethoxy-2-hydroxy-3: 4: 6-trimethoxy-acetophenone obtained synthetically, thus confirming again the above constitution of (B) and of hibiscitrin.

Ab initio and DFT methods have been employed to study the hydrogen bonding ability of formamide, urea, urea monoxide, thioformamide, thiourea and thiourea monoxide with one water molecule and the homodimers of the selected molecules. The stabilization energies associated with themonohydrated adducts and homodimers’ formation were evaluated at B3LYP/6-311++G** and MP2/6-311++G** levels. The energies were corrected for zero-point vibrational energies and basis set superposition error using counterpoise method. Atoms in molecules study has been carried out in order to characterize the hydrogen bonds through the changes in electron density and laplacian of electron density. A natural energy decomposition and natural bond orbital analysis was performed to understand the nature of hydrogen bonding.