Vibrational Spectroscopic and Density Functional Theory Studies on N,N-Diglycydal-5,5-dimethylhydantoin

Vibrational Spectroscopic and Density Functional Theory Studies on N,N-Diglycydal-5,5-dimethylhydantoin

M. Adityaa and V. Venkatesanb*,

aDepartment of Physics, IIT Madras, Chennai-600 036, IndiabResearch & Innovation Centre, IIT Madras Research Park, Chennai-600 113, IndiaAbstract

Conformations of N,N-Diglycydal-5,5-dimethylhydantoin (DDH) were studied using both experimental and computational methods. We have recorded the infrared spectrum of DDH in the region of 650-4000 cm-1. As a result of these experiments, infrared spectra of the ground and higher energy conformers of DDH are reported, for the first time. The experimental studies were supported by ab initio computations performed at B3LYP level, using a 6(31++G** basis set. Computationally, thirty optimized structures of DDH were identified. The computed frequencies at the B3LYP level for the A1, A2, A3 and A4 conformers, were found to compare well with the experimental infrared frequencies, leading to a definitive assignment of the infrared features of DDH, for these conformers. Key words: hydantoin; conformers; vibrational spectroscopy; Density functional theory1. Introduction

Bonding agents are important components of rocket solid propellants which interface with the surface of the oxidizer such as ammonium perchlorate and is chemically reacted to the polymer binder such as hydroxyl terminated polybutadiene (HTPB) network during the propellant cure process. They form a small component (0.3%-0.5%) of the overall propellant formulation, but is the most important ingredient in the formulation since it affects processing, mechanical properties, ballistics, safety, aging, temperature cycling and insensitive munitions (IM) propellant characteristics. N,N-Diglycydal-5,5-dimethylhydantoin (DDH) is a bonding agent in a propellant system. The interaction between DDH and oxidizer and polymer is not yet well understood in the literature. Before studying the interaction, it is necessary to understand the structure of DDH at the molecular level. Vibrational spectroscopy such as infrared and Raman is well known technique for molecular structure analysis. Infrared spectroscopy is also used both qualitative and quantitative identification. An unknown material can be determined by comparing its infrared spectrum to the spectra of known compounds. Also IR spectroscopy can be used to interpret the structure of an unknown compound. The spectral lines are produced by the absorption of the incident radiation by the vibrational modes of the functional groups in a molecule. Raman spectroscopy is a method of determining the modes of molecular motions, especially vibrations. It is predominantly applicable to the qualitative and the quantitative analysis of the covalently bonded molecules. It is based on Raman scattering which is the inelastic scattering, during which there is an exchange of energy between the photon and the molecule. Also, no computational work has been done on the structures and energies of DDH. We have therefore, for the first time, studied the conformers of DDH, using infrared spectroscopy and ab initio computations. The schematic structure of DDH is as follows:

2. Objective of Study

To theoretically find out different conformers of the above molecule using the theory B3LYP and the basis set 6-31G** & 6-31++G** To calculate their Boltzmanns distribution at room temperature using their energies and Their computed infrared and raman frequencies with experimental findings. 3. Experimental FTIR spectrum was performed using a Thermo Nicolet FTIR iS5 with the spectral resolution of 4 cm-1 in the region of 650-4000 cm-1. DDH sample synthesized from CLRI, Chennai. 4. Computational To support the experimental work on hydantoin complex, computations were carried out using density functional theory calculations. Computations were done to obtain molecular properties such as energies, structures, atomic charges on the various atoms and vibrational frequencies of the molecules. A detailed treatment on the density functional theory computational procedure is given in various books.(a) Geometry optimization of the structure At the outset, a structure corresponding to a minimum on the potential surface is obtained following a geometry optimization. A geometry optimization begins at the molecular structure specified as input, and steps along the potential energy surface. The energy and gradient are first computed at the point on the potential surface corresponding to the initial geometry. This information is used to determine how far and in which direction the next step is taken to improve the geometry. At the minimum (or more generally a stationary point), forces will be zero. In the case of the Gaussian program, the optimization was achieved when the forces, the root mean square of the forces, the calculated displacement and the root mean square of the displacement for the next step are below preset threshold values. All calculations were done using a 6-31G** and 6-31++G** basis sets. Optimization was carried out at density functional method (B3LYP) level of theory to locate structures corresponding to minima on the potential surface. The B3LYP uses the Becke three-parameter non-local exchange functional with non-local correlation of Lee et al. In these computations, all the molecular parameters defining the conformers were left free for the optimization. Various properties of the conformers such as energy, charges on the atoms, vibrational frequencies and dipole moments were computed at the B3LYP/6-31G** and B3LYP/6-31++G** levels of theory.(b) Vibrational frequency of the conformerFrequency calculations were done for all the complexes corresponding to the optimized geometries. These calculations helped us to ascertain that the structure of the conformer that we obtained following the optimization procedure did indeed correspond to a minimum, by ensuring that all the frequencies were positive. Only structures that correspond to minima are reported. The vibrational frequency calculations also helped us to assign the spectral features observed in our FTIR experiments.5. Results and DiscussionWe found thirty two minima using a B3LYP/6-31G** level of theory. Among the thirty two structures, two structures are converged to one of the optimized other structure in B3LYP/6-31++G** level of theory. The optimized structure of DDH is as follows:A1 A2 A3A4

A5 A6A7A8

A9A10A11A12

A13 A14 A15 A16

A17A18 A19

A20

A21 A22 A23 A24

A25 A26 A27 A28

A29

A30

A31

A32The numbering of atoms is as follows:

The optimized geometrical parameters of DDH using B3LYP/6-31++G** level is listed in Table 1.

Table 1: Dihedral angles of optimized DDH conformers using B3LYP/6-31++G** level of theory.ConformersDihedral Angle (in degrees)

3291045811291012581115

A181-95-157-94

A28289-157-93

A379-114-157144

A48666-157144

A58696-15735

A682-86-15732

A71049097-93

A8105-9198-94

A9-105-90-19-93

A10-10589-20-93

A11102-11397143

A121036698143

A13-102-93103-93

A14-10488102-94

A15102969536

A16-97-112104143

A17104-869734

A18-10167103143

A19-10298-1636

A20-10298-1636

A21-106-85-1935

A22-919610536

A23-7990-136-93

A24-78-92-136-93

A25-9297-7825

A26-98-8510334

A27-78-112-138143

A28-7767-139145

A29-77-85-13636

A30-77-85-13636

A31106-96-16-93

A3210488-14-93

The relative energy values of DDH using B3LYP level is listed in Table 2. Table 2: Relative energy values of conformers using B3LYP level of theory. Conformers(E (kcal/mol)% Population

6-31G**6-31++G**

A100.0333.52

A20.01032.00

A30.550.5213.98

A40.750.6610.96

A51.161.55< 5

A61.291.57< 5

A71.471.13< 5

A81.491.17< 5

A91.491.59< 5

A101.521.59< 5

A111.841.49< 5

A122.131.73< 5

A132.131.76< 5

A142.271.85< 5

A152.352.47< 5

A162.582.15< 5

A172.682.60< 5

A182.762.28< 5

A192.783.24< 5

A202.793.24< 5

A212.833.25< 5

A223.173.15< 5

A233.33< 5

A243.333.08< 5

A253.614.71< 5

A263.693.49< 5

A273.753.55< 5

A284.454.07< 5

A294.84.83< 5

A304.884.83< 5

A314.964.90< 5

A325.215.16< 5

As can be seen in Table 2, the energy ordering of conformers are not changed significantly. The change in the dihedral results in different conformational isomeric structures. These conformers are in equilibrium with each other. This equilibrium is determined by the energy difference between different conformational structures. Hence the structure with the least energy i.e the most stable structure is the dominant one. The population of different conformers follows a bolzmann distribution.

The left hand side is the equilibrium ratio of conformerito the total.is the relative energy of thei-th conformer from the minimum energy conformer.is the relative energy of thek-th conformer from the minimum energy conformer.Ris the molar ideal gas constant equal to 8.31J/(molK) andTis the temperature inkelvins(K). The denominator of the right side is the partition function. A1 and A2 conformers did not differ much in energy values, which showed most stable than those of other conformers. At room temperature structures A1 and A2 comprise 65% of the total population since their energies are the lowest. The most stable structures have the highest population and hence the frequencies corresponding to their molecular vibrations are the dominant ones. The most dominant frequency in the IR spectrum is due the carbonyl stretch which is at 1766 cm-1. To compare the experimental frequencies, we took the computed frequencies of the first four highest populated conformers. The experimental and computed infrared frequencies of DDH conformers are listed in Table 3.Table 3: Experimental and computed infrared frequencies of DDH conformers. Computational (cm-1)Experimental

(cm-1)Approximate mode description

A1A2A3A4

595(24)594(27)596(18)593(27)5 member Ring deformation (in the plane)

627(19)629(17)629(16)626(15)5 member Ring deformation (in the plane)

667(15)671(16)660(11)671(8)5 member ring deformation in the plane

--702(10)694(8)5 member ring deformation in the plane

764(11)763(10)765(13)-774(w)5 member Ring deformation (out of the plane)

784(13) & 818(24)784(17) & 818(35)783(16) & 789(20)783(14)808(w)3 member epoxy ring deformation

857(24), 865(28) 871(20)857(10), 865(22) 868(34)854(26), 860(13) 879(21)853(11), 861(17) 884(37)857(m)C-H bond deformation

948(42) 964(13)950(25) & 963(26)961(46)949(10) & 956(47)945(m)C-H bond deformation

998(36) 1077(30)1000(26) & 1086(22)1106(32) & 1110(18)1086(15), 1099(29) & 1111(28)1110(s), 1160(s)C-H bond deformation

1320(87), 1338(74), 1372(39), 1386(42) 1402(21), 1426(46), 1443(179), 1457(123), 1466(53), 1478 (108)1319(101), 1342(18), 1373(54), 1383(69), 1402(21), 1425(47), 1445(183), 1457(103), 1465(49), 1477(126)1318(94), 1343(73), 1367(28), 1376(48), 1401(22), 1427(41), 1445(209), 1459(39), 1465(22), 1477(215)1323(111), 1366(57), 1378(48), 1402(19), 1425(55), 1441(241), 1458(25), 1463(24), 1476(184)1260(v.s), 1350(w), 1390(w), 1450(vs) C-H bond deformation in methyl group

1512(10)1511(10) & 1536(13)--1490(w)C-H bond deformation in methyl group

1769(755)1770(738)1768(767)1770(727)1710(vs) & 1770(s)C-O bond deformation & C-N bond deformation along their length

3054(38) & 3085(12)3054(37) & 3084(11)3052(43) & 3064(20)3057(34) & 3059(14)2880(w), 2940(w), 3000(w)C-H bond deformation along its length

Intensity in brackets. (vs-very strong, s-strong, m-moderate, w-weak, v.w-very weak)

The computed Raman frequencies of conformers are listed in Table 4. Table 4: Computed Raman frequencies of DDH conformers using B3LYP/6-31++G** level of theory. Computational (cm-1)Approximate mode description

A1A2A3A4

627(14)629(13)629(14)626(12)5-member ring deformation in the plane

1284(10) & 1285(34)1284(43)1285(29) & 1288(32)1285(31) &1290(27)C-C bond elongation in 3 membered ring

1452(11) & 1466(11)1452(9) & 1465(16)1453(13)1451(11) & 1458(13)C-C bond elongation

1823(25)1823(23)1824(24)1822(24)C-O bond elongation

3049(16), 3054(35) & 3056(301)3049(12), 3054(49) & 3056(285)3052(132), 3057(211) & 3064(128)3049(25), 3055(92), 3057(174) & 3059(183)C-H bond elongation in methyl group

3085(109)3084(109)--C-H bond elongation

3102(139)3103(140)3102(147)3102(145) & 3101(142)C-H bond elongation for the carbon in the 3 membered ring

3110(110)3109(98)3107(45)3110(45)C-H bond elongation for the carbon in the 3 membered ring

3124(97) &3125(48)3125(48) & 3127(89)3125(51)3125(50)C-H bond elongation for the carbon in the 3 membered ring

3131(44)3131(45)3132(38) & 3133(58)3131(44) & 3134(56)C-H bond elongation in methyl group

3135(58) & 3140(57)3136(62) & 3140(54)3136(63) & 3141(57)3137(56) & 3139(56)C-H bond elongation for the carbon in the 3 membered ring

3147(25) & 3151(62)3146(27) & 3150(63)3150(64)3149(64)C-H bond elongation

--3156(35)3156(30)C-H bond elongation for the carbon in the 3 membered ring

3192(97)3193(100)3190(99) & 3190(98)3190(99) & 3192(100)C-H bond elongation for the carbon in the 3 membered ring

3206(53)3204(54)--C-H bond elongation for the carbon in the 3 membered ring

6. ConclusionsWe have recorded the infrared spectrum of DDH. Spectral features of A1, A2, A3 and A4 conformers are observed. The experimental vibrational frequencies of the above conformers are close to the calculated value at the B3LYP/6-31++G** level of theory. This work has lead to the assignment of various features of DDH. 7. References1. Di-methyl hydantoin bonding agents in solid propellants, John P.Consaga, DTIC report No. ADD013097, 1980.2. Raman: theory and instrumentation, Kit Umbach Dept. of MS&E.3. Introductory Raman Spectroscopy, J. R. Ferraro, C. W. Brown, Academic Press, 2003.4. Willard et al. Instrumental methods of Analysis, 7th edition, Wadsworth Publishing Co., Belmont, CA 1998. HYPERLINK "http://www.lookchem.com/300w/2010/0619/15336-81-9.jpg" \t "_blank"

FTIR spectrum of DDH in the region of 650-4000 cm-1.