CONFORMATIONAL ISOMERISM IS A FORM OF STEREOISOMERISM IN WHICH THE ISOMERS CAN BE INTERCONVERTED EXCLUSIVELY BY ROTATIONS ABOUT FORMALLY SINGLE BONDS. SUCH ISOMERS ARE GENERALLY REFERRED TO AS CONFORMATIONAL ISOMERS OR CONFORMERS AND SPECIFICALLY AS ROTAMERS; IN THE SENSE THAT THERE EXISTS A ROTATIONAL ENERGY BARRIER THAT NEEDS TO BE OVERCOME TO CONVERT ONE CONFORMER TO ANOTHER. THE FORCES UNDERLYING THE BARRIER TO ROTATION AROUND FORMALLY SINGLE BONDS HAVE BEEN THE FOCUS OF NUMEROUS STUDIES [1-2].THERE ARE STRONG EVIDENCES POINTING OUT THAT HYPERCONJUGATION IS INVOLVED, WITH APPRECIABLE IMPORTANCE, IN A REPRESENTATIVE NUMBER OF SITUATIONS [3]. ROTATIONAL BARRIERS PLAY A FUNDAMENTAL ROLE IN THE MOLECULAR SIMULATION. WITH THIS REGARD, ALL CONFORMERS OF THE ENOL FORM OF DIPIVALOYLMETHANE (DPM) COMPOUND HAVE BEEN STUDIED BY CONSIDERING THE ROTATION OF THE T-BUTYL GROUPS, IN BOTH SIDE OF HYDROXYL AND CARBONYL, ABOUT THE C-C BONDS.THE RELATIVE ENERGIES OF ALL CONFORMERS AND ALSO THE BARRIER HEIGHTS FOR INTERNAL ROTATIONS HAVE BEEN EVALUATED.ALL CALCULATIONS IN THIS STUDY HAVE BEEN PERFORMED BY THE GAUSSIAN 03W AND NBO 5.0 PROGRAM PACKAGES. THE FULL GEOMETRY OPTIMIZATIONS FOR THE ENOL FORMS OF DPM HAVE BEEN CARRIED OUT USING B3LYP LEVEL OF DENSITY FUNCTIONAL THEORY (DFT), AT 6-31G (D), 6-311G (D, P) AND 6-311++G (D, P) BASIS SETS. THE VIBRATIONAL FREQUENCIES HAVE BEEN ALSO CALCULATED AT THESE LEVELS. ALL CALCULATIONS SHOW THAT ONLY TWO CONFORMATIONS OF THE ENOL FORMS ARE STABLE. IN THE MOST STABLE FORM, ONE T-BUTYL GROUP IS IN STAGGERED POSITION WITH ITS NEARBY C-O SINGLE BOND (HYDROXYL SIDE), AND THE SECOND T-BUTYL GROUP IS IN THE ECLIPSE POSITION RESPECT TO ITS NEARBY CARBONYL GROUP. THE OTHER LESS STABLE ENOL CONFORMER IS THE FORM IN WHICH BOTH T-BUTYL GROUPS ARE IN THE STAGGERED POSITION WITH RESPECT TO THE C-O AND C=O BOND. ALL OTHER ENOL CONFORMATIONS DO NOT SHOW ANY STABILITY AND TURN TO THE TWO AFOREMENTIONED STABLE CONFORMERS, UPON FULL OPTIMIZATION. THE REASONS OF THESE STABILITIES HAVE BEEN EVALUATED BASED ON THE MOLECULAR STRUCTURE, THE INTRAMOLECULAR HYDROGEN BONDING, HYPERCONJUGATION INTERACTIONS, ATOMIC NATURAL BOND ORBITAL (NBO) CHARGES, SECOND ORDER PERTURBATION THEORY ANALYSES OF DONOR-ACCEPTOR CHARGE TRANSFER IN NBO BASIS, DIPOLE MOMENTS AND POLARIZABILITIES.

INTRODUCTION CURRENTLY, THE PHOSPHORAMIDATES CHEMISTRY IS OF GREAT INTEREST DUE TO ITS WIDE APPLICATIONS IN VARIOUS AREAS OF SCIENCE [1, 2]. THERE HAVE BEEN MANY EFFORTS FOR THE SYNTHESIS, CHARACTERIZATION AND STRUCTURAL INVESTIGATIONS ON THESE COMPOUNDS [3, 4]. THE QUANTUM CHEMICAL COMPUTATIONS WERE CONDUCTED ON PHOSPHORAMIDATES TO PREDICT THEIR STRUCTUREAL PARAMETERS AND TO COMPARE THE CALCULATED RESULTS WITH THE EXPERIMENTAL ONES [5, 6].COMPUTATIONAL DETAILS THE STRUCTURE OF A NEW PHOSPHORIC TRIAMIDE WITH FORMUAL 4-CH3-C6H4S (O) 2NHP (O) (NHCH2C4H5) 2 WAS OPTIMIZED IN THE GAS PHASE USING GAUSSIAN 98 PROGRAM [7] AT DFT (B3LYP AND B3PW91) LEVEL OF THEORY WITH STANDARD 6-31+G** BASIS SET. THE NATURAL BOND ORBITAL (NBO) CALCULATIONS WERE PERFORMED TO OBTAIN THE ATOMIC HYBDIDIZATIONS.

AS A GAS-TO-LIQUID (GTL) REACTION, FISCHER–TROPSCH (FT) SYNTHESIS CAN PRODUCE HYDROCARBONS USING SYNGAS (CO AND H2) DERIVED FROMCOAL, NATURAL GAS, OR BIOMASS.1 SUPPORTED COBALT CATALYSTS ARE PREFERRED DUE TO THEIR HIGH ACTIVITY...

IN METHYLATED MOLECULES, THE METHYL GROUP CAN ROTATE AROUND A SINGLE BOND SO THAT DURING A FULL 360° ROTATION IT THRICE ENCOUNTERS AN UNSTABLE AND A RELATIVELY STABLE CONFORMATION.THE ENERGY DIFFERENCE BETWEEN THESE TWO CONFORMERS GIVES RISE TO POTENTIAL BARRIER TO THIS ROTATION. THE QUANTUM MECHANICAL NATURE OF THIS HINDERED ROTATION IN MOLECULES WAS RECOGNIZED IN THE EARLY YEARS OF QUANTUM THEORY [1]. CONFORMATIONAL ANALYSIS IS A FUNDAMENTAL SUBJECT IN ORGANIC AND PHYSICAL CHEMISTRY. HOWEVER, NOT EVERY QUESTION CONCERNING THE CONFORMATION ANALYSIS IS SIMPLE TO ANSWER. A B -DICARBONYL COMPOUND, WITH AT LEAST ONE ALPHA PROTON, PREDOMINANTLY EXISTS AS CONJUGATEDCIS -ENOL FORM [2]. OVER THE YEARS, THE KETO-ENOL TAUTOMERIC EQUILIBRIUM AND THE STRUCTURE OF BOTH KETO AND ENOL FORMS OF B-DICARBONYL HAVE BEEN THE SUBJECTS OF INTENSIVE STUDIES USING A LARGE VARIETY OF DIFFERENT METHODS [2]. ACETYLACETONE (AA) IS A WELL-KNOWN MEMBER OF B-DICARBONYL COMPOUNDS. A FULL CONFORMATIONAL ANALYSES OF AA, WITH SPECIAL ATTENTION ON THE CONFORMATIONS OF TWO METHYL GROUPS IN THE SIDE OF CARBONYL AND HYDROXYL GROUPS, IN ITS ENOL FORM, HAVE BEEN THE SUBJECT OF THE PRESENT WORK BY MEANS OF AB INITIO AND DENSITY FUNCTIONAL THEORY (DFT) CALCULATIONS. ALL QUANTUM MECHANICAL CALCULATIONS HAVE BEEN PERFORMED USING GAUSSIAN 03W PACKAGE PROGRAM AND NBO 5.0 SOFTWARE. ALL POSSIBLE ENOL CONFORMATIONS OF AA HAVE BEEN FULLY OPTIMIZED AT HF AND HYBRID DENSITY FUNCTIONAL B3LYP LEVELS, USING 6-31G (D), 6-311G (D, P) AND 6-311++G (D, P) BASIS SETS. THE VIBRATIONAL FREQUENCIES HAVE BEEN CALCULATED AT THE SAME LEVEL. ALSO VISUAL ANALYSES HAVE BEEN MADE WITH THE GAUSSVIEW 3.0 PACKAGE. THE OBTAINED RELATIVE STABILITIES IN ALL LEVEL OF CALCULATIONS SHOW THAT, IN THE MOST STABLE ENOL FORM OF AA, THE HYDROXYL AND THE NEARBY METHYL GROUPS ARE IN THE STAGGERED POSITION, BUT THE CARBONYL AND ITS NEARBY METHYL GROUPS ARE IN THE ECLIPSE POSITION RESPECT TO EACH OTHER. THE DETAILED REASONS HAVE BEEN INVESTIGATED BY COMPARING THE MOLECULAR STRUCTURE, THE INTRAMOLECULAR HYDROGEN BONDING, VARIETIES OF HYPERCONJUGATION INTERACTIONS, ATOMIC NATURAL BOND ORBITAL (NBO) CHARGES, SECOND ORDER PERTURBATION THEORY ANALYSES OF DONOR-ACCEPTOR CHARGE TRANSFER IN NBO BASIS, DIPOLE MOMENTS AND POLARIZABILITIES.

BILIRUBIN IS A COMPOUND GENERATED BY METABOLISM OF HEMOGLOBIN AND OTHER IRON PORPHYRIN CONTAINING ENZYMES IN MAMMALS [1]. IF THE CONCENTRATION OF BILIRUBIN EXCEEDS A CERTAIN LEVEL IN THE BLOOD, SOME LIVER DISEASES MAY BE OCCURRED. IN THE TREATMENT OF THESE DISEASES, THE REMOVAL OF THE EXCESS BILIRUBIN FROM THE BLOOD IS THE MOST IMPORTANT MATTER. ONE OF THE PROPOSED METHODS FOR THE REMOVAL OF THE EXCESS AMOUNT OF BILIRUBIN IS ITS EGESTING FROM THE BLOOD USING NANOSTRUCTURED COMPOUNDS, SUCH AS CARBON NANOTUBES (CNTS) [2]. THE PURPOSE OF THIS WORK IS A THEORETICAL STUDY ON THE ADSORPTION OF BILIRUBIN ON SOME NANOPARTICLES, SUCH AS (5, 5) SINGLE WALL CNT (SWCNT), FULLERENE AND GRAPHENE. SWCNT (5, 5) HAS BEEN USED BECAUSE OF ITS MODERATE REACTIVITY AND STABILITY [3]. ALL THE CALCULATIONS IN THIS STUDY HAVE BEEN CARRIED OUT USING GAUSSIAN 03 AND NBO 5.0 PROGRAMS. THE AB INITIO HF AND B3LYP METHOD OF DENSITY FUNCTIONAL THEORY (DFT) HAVE BEEN USED FOR ALL CALCULATIONS, USING6-31G AND 6-31G (D) BASIS SETS. AFTER FULL OPTIMIZATIONS, SEVERAL THEORETICAL AND THERMODYNAMIC PARAMETERS SUCH AS MOLECULAR STRUCTURES, NBO CHARGES, DONOR-ACCEPTOR ORBITAL INTRACTIONS, HOMO AND LUMO ENERGY SURFACES AND THEIR BAND GAPS, DIPOLE MOMENTS, CURVATURE EFFECTS, POLARIZABILITIES, BINDING ENERGIES, DG, DH AND DS HAVE BEEN INVESTIGATED FOR BILIRUBIN ADSORPTION ON THE MENTIONED NANOSTRUCTURE SURFACES. THE CALCULATIONS SHOW THAT BILIRUBIN ADSORPTIONS ON ALL TARGET SURFACESARE THERMODYNAMICALLY FAVORABLE, BUT THE INTERACTIONSARENON-COVALENT (Π-INTERACTION), BECAUSE THE CALCULATED ADSORPTION ENERGY VALUES ARE NOT SIGNIFICANT. THE HIGHEST VALUE FOR THE BINDING ENERGIES BETWEEN THE BILIRUBIN AND THE TARGET NANOSTRUCTURES IS OBTAINED FOR GRAPHENE, WHILE THE ADSORPTION OF BILIRUBIN ON THE FULLERENE LEADS TO THE LOWEST BINDING ENERGY. THIS SHOWS THAT INCREASING THE CURVATURE OF THE ABSORBENT DECREASES THE AMOUNT OF ADSORPTION. ALSO, COMPUTED HOMO-LUMO BAND GAPS FOR THE BILIRUBIN, STUDIED NANOPARTICLES AND THEIR COMPLEXES SHOW THAT THE CHARGE TRANSFER IN THE COMPLEXES ARE MORE FAVORABLE THAN IN BILIRUBIN OR SELECTED NANOSTRUCTURE. OTHER MENTIONED PROPERTIES FOR BILIRUBIN ADSORPTION ON (5, 5) SWCNT, FULLERENE AND GRAPHENE HAVE BEEN INVESTIGATED AND COMPARED WITH EACH OTHER AND WITH OTHER WORKS, AND THE RESULTS HAVE BEEN DISCUSSED.