iMlianJ. Phys. 78(12), 1391-1393 (2004) o A itU

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Electronic spectrum of octaazacubane (Ns)

R S Prasad* Department of Physics, Mugi^tfh University, Bodh-Gaya-824 234, Bihar, India

and K Kumar Depaitment of Physics, Ramgarh-821 110, Bihar. India

E-mail j [email protected]

Received 24 May 2004, accepted 16 Seplemher 2004

AiMtract : MRINDCVS calculation augmented by singly excited configuration interaction was perfomied on octaazacubane. We report ionization potentials, singlet and triplet excitations and the singlet-inplet splittings in octaazacubane. The importance of outer (Rydbeig) atomic orbitals is stressed and it is found that none of the transitions leads to an excited stale with considerable Rydberg character.

Keywords ; Octaazacubane, ionization potentials, electronic spectrum.

PACSNos. : 31.15.Ne, 33.15.Ry, 33.20.Lg

By use of ab initio self-consistent-field (SCF), coupled thermodynamically unstable, pathways for decomposition duster and many-body perturbation theory (MBPT) to ground state N2 are forbidden by orbital symmetry. methods, the octahedral nitrogen molecule Ng is found to Therefore, it is possible that N4 and Ng can be observed be in a metastable state, corresponding to local minimum experimentally [1]. on its potential energy surface [1]. The potential existence Lauderdale et al [1] argue that while each of these of more exotic metastable species offers insight into three molecules (N 4 , Ne, and Ng) has been studied in the nature of the chemical bond. For example, the possible past 13- 1 5 ], the effects of electron correlation have not metastable molecules include three forms of N3H3 been thoroughly explored. Furthermore, little attention has analogous to propane, cyclopropane and ozone been paid to characterizing minimum by the calculation (experimentally known [2]) and the cubane analogue Ng. of second derivatives. Without this critical piece of

1 j»iiA»nifll«» et al [1] studied the bonding and stability information, any discussion of thermodynamical stability of Ng. They opine that synthesis of metastable molecules is doubtful. would offer a potential route towards the storage of laige Trinquier et al [8 ] looked at tetrahedral X4 and amount of eneigy. A great deal of efforts has been cubane Xg structures of N, P, and S using SCF focussed on increasing density by producing highly strained pseudopotential calculations. They found Ng to be much molecular systems. One way of further increasing the less stable (thermodynamically) than 2 N4. They concluded eneigy of these systems is to replace each CH fragment that all Xg systems are generally less stable. Engelke et with an N atom. In the limit of full replacement, nitrogen al 115] optimized the Ng geometry at the SCF level using analogues of tetrahedrane N4 and cubane Ng are obtained. STO-3G, 4-31G, and 4-31G* basis sets. They found Ng While one would expect these molecules to be configuration to be stable minimum with each basis set

Comgpanding AuUior © 2004 lACS 1392 R S Prasad and K Kumar by determining the harmonic frequencies and concluded oscillator strengths and the symmetries of the excited that since the lowest mode was relatively strong, the states. The last colunui of Table 2 contains the percentage molecule would be difficult to deform. No structures Rydberg character of the states. Table 3 gives transmon were deteilhined and characterized at the correlated level. energies, symmetries of the excited states and singlet- However, Lauderdale et ctfHl] investigated octaazacubane triplet splittings. The last column of Table 3 give.s the (Ng) including correlated predictions of vibrational percentage Rydberg character of the states. frequencies. Tlible 2. Lowest singlet excitations in octaazacubane. All energies are in e\' Leininger et al [16] using ab initio molecular electronic Molecule MRINDO/S spectrum methods examined three isomers of Ng. In %Rydberg characuT addition to the previously studied octaazacubane structure (Symmetry) Transition / Sym. of Type N (Og symmetry), they investigated a D2d strcture analogous cneigy excited state to cyclooctatetraine, and a planar bicyclic form analogous N, 5,842 0.000 Ti. n <3^ 0 to pentalene. Han et al [17] investigated, in addition to (O*) 5.847 0.000 0 Oh symmetry, two new structures of Du and C2, syirmietry, 5.850 0,000 ft* 0 without N=N double bonds. 5.857 0.000 n —> 0* 0 According to the above discussion, only the stability 5.867 0.000 Tu n ^ 0 and structure of octaazacubane were searched from 5,984 0.037 Tu n-¥ n* 0 different theoretical point of view. But the electronic 6.066 0.036 A\, a* 0 spectrum of octaazacubane is not found in the literature. 6.102 0.026 Tx, n* 0 This stimulated us to take into account of the electronic 6.113 0.076 Ajj, a* 0 spectrum of octaazacubane. In this work, we have studied 6.166 0.001 n . ft* 0 the spectrum of octaazacubane (Oh symmetiy) at the SCF 6.174 0.010 AtM n—¥0* 0 level of theory using STO basis set. The method employed 6.361 0.000 /I —► 1 in this work, can be found in Ref. [18]. 6.363 0.000 Ti. n —> /r**' 1 The optimized geometry of Ng {R = 1.525 A°) which 6.407 0.000 Tu n-^ ft* 1 was determined and characterized at the correlated level 6.408 0.000 n~^ Cf* 1 by Lauderdale et al [1], was used in this work. 6.411 0.001 ^1. n a* 1 Tables 1-3 contain the computed results of 6.412 0.001 Aj, n ^ 0* 1 octaazacubane. Table 1 contains ionization potentials 6.508 0.000 n—> ft* 0 deduced from Koopmans theorem and the orbital 6.584 0.004 ' ‘a. n~~> 0* 0 symmetries. Table 2 contains singlet excitation energies, 6.600 0.000 ft* 0 Tkble 1, The lowest ionization potentials of octaazacubane (eV). 6.610 0.000 n-^ft* 0 6.621 0.002 n 0* 0 Molecule MRINDO/S -'a. 6.896 0.069 ft * 0 (Symmetiy) 1.R Symmetry Type ^a. n--¥ 0* 0 of orbital 7.087 0.002 ' ‘a, 7.101 0.011 ft * 0 N. 11.197 ^a. 7.393 0.000 0* 1 (O h) 11.199 a 13.027 hx n 7.436 0.001 n - * ft* 0 13.040 ajM cr 14.385 n Ionization potentials : 14,404 htt n The first ionization potential of octaazacubane is 15.811 a interpreted as the removal of an electron from a x orbital 17.496 flu a while the seemtd one as die removal of an electron from 18.425 a «»# a O'orbital. A conqrarison of these two iorization potentials 18.450 fht n 22.874 flZi a reveals that both ;r and rr electrons have af^ximately 22.878 0 the same tendency to leave the molecule. Again, conquring 31.390 0 the third and the fourth ionization potentials, a similar 31.394 aiM a tendency is found fOT both x and or electrons to leave the Electronic spectrum of octaazacubane (Ns) 1393 t^blc 3- Triplet excitations in ocUazacubane. All enei^ies are in eV. Triplet excitations : Molecule MRINDO/S %Rydbcrg The lowest triplet transition in octaazacubane is assigned character as n -» «■* at 5.375 eV. This triplet is separated from (Symmetry) lYansition S-T Sym. of Type N its corresponding singlet by an energy of 0.609 cV. As to cneixy split excited state the singlet-triplet splittings, the splittings for n —» N* 5.375 0.609 transitions are higher by about twice the splittings for n (Oh) 5.377 0.736 7'., /I —> —> a* ones. The n -* rr* transitions in octaazacubane 5.510 0.332 T2u o* exhibit intensity. All the triplet excitations are entirely 5.546 0.556 hi -4 ;r* valence shell type because they have zero percent Rydberg 5.555 0.302 n-^ o* character. We are unable to compare our results due to the lack of any other relevant theoretical or experimental 5.613 0.237 A I, n~^ JT* 5.617 0.230 n-^ (7* i 0 data in the literature. 5.628 0.239 hi n -4 CT* ; 0 The electronic spectrum of octaazacubane has not yet been observed or theoretically calculated in the literature. molecule. This in turn, implies that the n ;r* and n —> Our calculation reveals that the spectrum of octaazacubane cr* transitions would be of comparable intens^ in is compact. Nevertheless, the structural features of the octaazacubane. This aspect of the octaazacubane spectrum spectrum of octaazacubane are well pronounced. We hope is discussed in the succeeding section. that the present results would be helpful in futher studies.

Acknowledgment Singlet excitations : The calculations reported here were performed at the The lowest singlet excitation in octaazacubane is predicted computer centre, Banaras Hindu University, Varanasi. The at 5.842 eV. Thus, the octaazacubane spectrum cannot be helping attitude of the staff of this centre is gratefully expected in the visible or in the near-ultraviolet, for it acknowledged. appears in the medium ultraviolet under the approximation References used in this work. The first absorption in octaazacubane [1] W J Lauderdale, J F Stanton and R J Bartlett J. Phys. Chem. 96 takes place at 5.984 eV. The transition corresponding to 1173 (1992) this absorption is assigned as n -* ft* which has an [2J D H Magers, £ A Salter, R J Bartlett, C Salter, B A Hess and L J Schaady. Am. Chem. Soc. 100 3435 (1988) oscillator strength of 0.037. Several allowed transitions f3J J S Wright J. Am. Chem. Soc. 96 4753 (1974) are reported between 5.984 and 6.174 eV, the one at [4] T J Venanzi and J M Schulman Mol. Phys. 30 281 (1975) 6.133 cV is the most intense (f = 0.076) transition [5] G Trinquirc, J P Malrieu and J P Daudey Chem. Phys. Lett. 80 552 throughout the spectrum of octaazacubane. Thus, this (1981) energy corresponds to the maximum absorption in [6] M M Francl and J P Chesick J. Phys. Chem. 94 526 (1990) octaazacubane. The transition corresponding to the [7] A Voglcr, R E Wright and H Kunkley Angew. Chem. Int. Ed. Eng. maximum absorptitm is assigned as n -* (T*. Allowed 19 717(1980) f8) T K Ha, R Cimiraglia and M T Nguyen Chem. Phys. Lett. 83 317 transitions are also found in the shorter wavelength side (1981) of die spectrum, n ft* transition at 6.986 eV has an I9J P Saxe and H F Schaefer (ffl) J. Am. Chem. Soc. 105 1760 (1983) oscillator strength of 0.069. A comparison of oscillator (lOJ H Huber, T K Ha and M T Nguyen J. Mol. Struct. 105 351 (1983) strength of this n n* transition with that of the [111 M Ramck J. Mol. Struct. 109 391 (1984) maximum absorbing n - ^ a* transition (f - 0.076) reveals [12] R Engclkc J. Phys. Chem. 93 5722 (1989) that these transitions are of comparable intensity and can [13] M T Nguyen J. Phys. Chem. 94 6923 (1990) be obsnved experimentally because they are adequately [14] R Engclke J. Phys. Chem. 94 6924 (1990) spaced and in addition, the fornwr is jr-polarized while [15] R Engelke and J R Stine J. Phys. Chem. 94 5689 (1990) the lattu' is y-polarized. [16] M L Leininger, C D Sherrill and H F Schaefer (HI) / Phys. Chem. 99 2324(1995) In the present calculation, we used an extended basis [17] A Tian, F Ding, L Zhang, Y Xie and H F Schaefer (Ul) J. Phys. set (which includes Rydbeig atomic orbitals into the Chem. 101 1946(1991) valence basis set), but we notice that tte spectrum of [18] R S Prasad and B N Rai Theor. Chim. Acta 77 343 (1990) octaazacubane is devoid of Rydbeig excitations.