The Trigonal Bipyramidal MN3M Species: A New Kind of Aromatic Complex Containing a Multiple-Fold Aromatic N3 Subunit

DOI: 10.1002/cphc.200500292

The Trigonal Bipyramidal MN3M Species: A NewKind of Aromatic Complex Containing aMultiple-Fold Aromatic N3 Subunit**Ying Li, Zhi-Ru Li,* Di Wu, Wei Chen, and Chia-Chung Sun[a]

1. Introduction

Aromaticity, a concept generally associated with organic com-pounds, results in exceptional geometric, energetic, and mag-netic properties.[1] Ever since Kekul(’s[2] intuitive idea on thestructure of benzene, aromaticity has been one of the mostvexing yet fascinating problems in chemistry. Aromatic chemis-try challenged both the theoretician and the synthesist, andhas provided one of the most fruitful interplays of theory andexperiment in chemistry.[3]

In recent years, the concept of aromaticity has been success-fully extended from traditional organic molecules to inorganicmolecules.[4, 5] In 2001, Li et al.[6, 7] investigated MAl4

� (M=Li,Na, or Cu), NaGa4

� , and NaIn4� species. They reported evidence

of aromaticity for these purely metallic systems, and thus theirfindings expand the aromaticity concept into the arena of all-metal species. More recently, Tsipis et al.[8] theoretically predict-ed a new class of cyclic copper(I) hydrides formulated as CunHn

(n=3–6), and indicated the aromatic character of these cyclichydrocopper(I) species.As early as the 1970s, Dewar[9,10] suggested the concept of

s aromaticity to explain the rather unusual properties of cyclo-propane. Recently, Alexandrova et al.[11] extended the s aroma-ticity concept to small s-aromatic alkali-metal clusters to ex-plain the relative stability of Li3

+ and Li3� ions.

The study of multiple-fold aromatic species is currently anactive topic. It is suggested that the possible electronic contri-bution to the aromaticity of a molecule should not be limitedto only one particular delocalized bonding system, as morethan one independent delocalized bonding system of a mole-cule can simultaneously satisfy the electron-counting rule andrender aromaticity. Lately, several papers have reported thecharacterization of the multiple-fold aromaticity of organic and

inorganic species. Prasang et al.[12] reported experimental andtheoretical research on two- and three-dimensional aromaticcompounds such as boranes, carboranes, and their isoelectron-ic derivatives. In 2002, Zhan et al.[13] proposed the orbital analy-sis approach of multiple-fold aromaticity for the square-planarAl4

2� structure, which can be determined by three independ-ent delocalized (p and s) bonding systems. Similarly, the struc-tural and electronic stability of the square planar Hg4

6� ionshould be attributed not only to p aromaticity due to the pres-ence of the two p electrons in the pp orbital, but also to s aro-maticity due to the occupation of the two four-center s-bond-ing orbitals.[14]

The hypothetical existence of polynitrogen clusters has beenthe object of several theoretical investigations because of theirpossible use as high-energy-density materials (HEDMs). So farmany stable structures of all-nitrogen clusters have been pre-dicted theoretically.[15–22] Several theoretical investigations onthe metal–polynitrogen species have also been reported re-cently.[23–25]

Three-membered rings represent the simplest potentially ar-omatic or antiaromatic systems. The simplest all-nitrogen ringis N3.

[26–28] There have been several investigations on the cyclicN3 radical

[29,30] and N3+ ring[31,32] , in which the latter is indicated

to be an aromatic species with two p electrons. However, no

[a] Dr. Y. Li, Prof. Z.-R. Li, Dr. D. Wu, Dr. W. Chen, Prof. C.-C. SunState Key Laboratory of Theoretical andComputational Chemistry, Institute of Theoretical ChemistryJilin University, Changchun 130023 (P.R. China)Fax: (+86)431-8945942E-mail : [email protected]

[**] M=Be, B, Mg, Al, Ca.

A new kind of aromatic trigonal bipyramidal MN3M (M=Be, B,Mg, Al, and Ca) species, with all real frequencies, is obtained atthe MP2/6-311+G(3d) level. The nucleus-independent chemicalshift values are �102.16 ppm for the N3

3� ring, and �74.09,�79.39, �65.06, �74.44, and �62.33 ppm (at the geometricalcenter of the trigonal bipyramid) for BeN3Be, BN3B, MgN3Mg,AlN3Al, and CaN3Ca, respectively. Molecular orbital analysis indi-cates that the regular triangular N3

3� ring and each MN3M spe-cies have three aromatic six-electron systems (p, sp, and ss) andexhibit threefold aromaticity. The CaN3Ca species has a very low

vertical ionization energy of 3.64 eV at the CCSD(T)/6-311+G(3d)level, which is even lower than the ionization energy (3.9 eV) ofthe Cs atom. Therefore, CaN3Ca can be considered as a new su-peralkali species. A further study on the CaN3CaCl molecule con-firms the superalkali characteristics of CaN3Ca. Two interestingphenomena are explored in the MN3M species: the delocalizedelectron cloud of the N3 subunit is elongated by two M cations,and the electron clouds of two M cations are distended by theN3

3� ring.

2562 B 2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim ChemPhysChem 2005, 6, 2562 – 2569

previous theoretical or experimental study on the electron-en-riched N3

3� ring has appeared. Triply charged species are ex-pected to be rather unstable in the gas phase due to large in-tramolecular Coulomb repulsion. Thus, complexation withcounterions is required to produce more stable species and isalso convenient for mass analysis and photodetachment ex-periments.[33]

Herein, we reveal the multiple-fold aromaticity of a new N33�

ring and the trigonal bipyramidal sandwich-like MN3M (M=Be,B, Mg, Al, Ca) species, and explore the interactions betweenthe N3 ring and the M atom. We find superalkali characteristicsof the CaN3Ca species, and confirm these characteristics by thestudy of the superalkali halide molecule [CaN3Ca]

+Cl� .

Computational Methods

The geometries of the N33� ring and the MN3M (M=Be, B, Mg, Al,

and Ca) species with all real frequencies were optimized at theMP2/6-311+G(3d) level of theory. The geometry of the N3

3� ringwas further studied by using the coupled-cluster method [CCSD(T)]with the 6-311+G(3d) basis set.

Nucleus-independent chemical shift (NICS) values for the N33� ring

and the MN3M (M=Be, B, Mg, Al, and Ca) species were calculatedby the GIAO-HF/6-311+G(3d)//MP2/6-311+G(3d) method. Themolecular orbitals (MOs) of the open-shell MN3M species were cal-culated by using the spin-restricted Hartree–Fock ROHF/6-311+G(3d) method. In all calculations, the <S2> values in MP2 wavefunctions were 0.7500 for the MN3M species. All of the MO dia-grams were made with the GaussView program. Natural bond orbi-tal (NBO)[34,35] analysis was also performed to provide insight intothe bonding nature of these species, and the vertical ionization en-ergies (VIEs) were obtained by the CCSD(T)/6-311+G(3d) methodfor the five MN3M species.All calculations were performed using the Gaussian 03 programpackage.[36]

2. Results and Discussion

2.1. Structural Characteristics of the N33� Ring and

MN3M Species

The choice of basis set is important in accurate quantumchemistry calculations. Therefore, the smallest species (BeN3Be)is selected as an example for studying the basis set effects.The geometry of the BeN3Be molecule is optimized at the MP2level with 6-311+G*, 6-311+G(2d), 6-311+G(2df), 6-311+G(3d), and 6-311+G(3df) basis sets. The behavior of L (the dis-tance between M and the N3 plane) with the five basis sets is

shown in Figure 1. The result (L=1.3548 I) from the 6-311+G(3d) basis set is very near to that (L=1.3546 I) from the 6-311+G(3df) basis set, while the length of L is underestimated

(1.3499 I) with the 6-311+G(2d) basis set and overestimated(1.3562 I) with the 6-311+G(2df) basis set, even though the 6-311+G(2df) basis set has ten more basis functions than the 6-311+G(3d) basis set. Thus, the 6-311+G(3d) basis set ischosen in following calculations and discussions.The optimized geometrical structures for the MN3M species

and the N33� ring are illustrated in Figure 2, and the geometri-

cal parameters of the five MN3M species are listed in Table 1.The MN3M species all have D3h trigonal bipyramidal structureswith the two M atoms above and below the regular triangularN3 ring, which can be considered as a kind of sandwich-likecompound. It is found that the length of L is correlated withthe atomic radii of M due to small differences (<0.1 I) of theN···N distance of N3 subunits among the five species. The orderof the atomic radii[37] of the M atom is 0.860 (B)<1.113 (Be)<1.431 (Al)<1.600 (Mg)<1.970 I (Ca), and the order of the dis-

Figure 1. Behavior of the distance L of BeN3Be with five basis sets at theMP2 level.

Figure 2. Optimized geometry of the D3h MN3M species and the N33� ring.

Table 1. Optimized structures, electronic-state symmetries, characteristic vibrational frequencies, wc [cm�1] , and corresponding IR intensities [kmmole�1] of

the MN3M (M=Be, B, Mg, Al, Ca) species.

Structural parameters Atomic radii of M [I] State wc IR intensitySpecies N···N [I] M···N [I] L [I] aN�N�N [8] MP2 B3LYP MP2 B3LYP

BN3B 1.617 1.599 1.298 60.0 0.860 2A2’’ 2116 462 7.2K104 1492BeN3Be 1.612 1.644 1.355 60.0 1.113 2A1’ 1595 779 4.7K104 3226AlN3Al 1.609 1.968 1.735 60.0 1.431 2A2’’ 1495 491 9.0K104 661MgN3Mg 1.603 2.023 1.798 60.0 1.600 2A1’ 1068 623 6.8K104 825CaN3Ca 1.528 2.231 2.049 60.0 1.970 2A1’ 798 460 3.6K104 6152

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Trigonal Bipyramidal MN3M

www.chemphyschem.org

tance L is 1.298 (BN3B)<1.355 (BeN3Be)<1.735 (AlN3Al)<1.798(MgN3Mg)<2.049 I (CaN3Ca). As shown in Figure 3, the lengthof L increases with the atomic radius of M.

In each MN3M species, all N···N distances are identical andslightly longer than that (1.516 I) in the regular triangular N3

3�

ring (see Figure 2). The order of the N···N distances is 1.617(BN3B)>1.612 (BeN3Be)>1.609 (AlN3Al)>1.603 (MgN3Mg)>1.528 I (CaN3Ca), which is opposite to the order of distance L.Clearly, the longer the length of L, the shorter the N···N dis-tance; that is, the less the influence of M, the stronger the N�N bond.The vibrational frequencies and modes were obtained by

the MP2/6-311+G(3d) method, based on the geometries opti-mized at the MP2/6-311+G(3d) level for the five MN3M spe-cies. As shown in Table 1, one of the vibrational modes produ-ces a significant peak in the IR spectrum for each MN3M spe-cies, which helps to experimentally identify the MN3M speciesand can be considered as the characteristic vibration. In thischaracteristic vibrational mode, the two M atoms move in-phase perpendicular to the N3 plane (see Figure 4). However,according to the MP2 results, this vibrational mode has a verylarge vibrational frequency and at the same time an extremelyhigh IR intensity, near 105 kmmole�1 for each MN3M species,which seems abnormally large. To test the credibility of theMP2 results, we recalculated the vibrational frequencies of theMN3M species by the B3LYP method and the results are also

listed in Table 1. Compared to the B3LYP results, the MP2method exaggerates the characteristic vibrational frequencyand corresponding IR intensity, which may be due to the wavefunction instabilities.

2.2. Multiple-Fold Aromaticity of the N33� Ring and

MN3M Species

2.2.1. Nucleus-Independent Chemical Shift (NICS)

NICS, proposed by Schleyer et al. , is an efficient and simple cri-terion for probing aromaticity, which is based on the negativeof the magnetic shielding computed at or above the geometri-cal centers of rings or clusters. Systems with (significant) nega-tive NICS values are aromatic, and systems with strongly posi-tive NICS values are antiaromatic. Nonaromatic cyclic systemsshould therefore have NICS values around zero. The more neg-ative the NICS value, the more aromatic the system.[38–41]

Herein, we calculated the NICS values at the geometrical cen-ters of the N3

3� ring and the trigonal bipyramidal MN3M spe-cies to provide a direct measure of aromaticity. As tabulated inTable 2, the N3

3� ring (D3h) has a large NICS value of

�102.16 ppm at the GIAO-HF//CCSD(T) level of theory, and theNICS values of the BeN3Be, BN3B, MgN3Mg, AlN3Al, and CaN3Caspecies at the GIAO-HF//MP2 level are �74.09, �79.39, �65.06,�74.44, and �62.33 ppm, respectively (�7.8 ppm for ben-zene[42]). The negative NICS values suggest the existence ofelectron delocalization and strong aromaticity in the N3

3� ringand the five MN3M species.Notably, the NICS values at the center of the N3 ring for all

MN3M species are smaller than that of the isolated N33� ring. In

addition, compared to the isolated N33� ring, the negative NICS

values extend to the two M cations after the introduction ofthe counterions (see Figure 5).From Figure 5, when d=0.0 I, the NICS value

(�102.16 ppm) of N33� is larger than that (�79.39 ppm) of theN3 subunit in BN3B. However, when d>0.4 I, for same NICSvalues the d value of the N3 subunit (with counterions) is largerthan that of the isolated N3

3� ring. For example, for a NICSvalue of about �50 ppm, d=0.7 I for the N3 subunit is largerthan d=0.5 I for the isolated N3

3� ring. This result shows that,due to the attractive interaction between the electron cloud ofthe N3 subunit and two M cations, the delocalized electroncloud of the N3 subunit is elongated, and consequently thespace of negative NICS values for the N3 subunit extends tothe two M cations.

Figure 3. Behavior of L with increasing atomic radius of M.

Figure 4. Characteristic vibrational mode of the MN3M species.

Table 2. Calculated NICS values [ppm] using the GIAO-HF//MP2 methodfor the MN3M (M=Be, B, Mg, Al, Ca) species and GIAO-HF//CCSD(T)method for the N3

3� ring.

CaN3Ca MgN3Mg BeN3Be AlN3Al BN3B N33�

NICS[a] �62.33 �65.06 �74.09 �74.44 �79.39 �102.16

[a] Calculated NICS at the geometrical center of the N3 ring.

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Z.-R. Li et al.


2.2.2. Multiple-Fold Aromaticity of the N33� Ring and

MN3M Species

To further understand the aromatic nature of the N33� ring, its

MOs were analyzed in detail. Figure 6a shows the nine valenceMOs of the regular triangular N3

3� ring, which are divided intothree types. The first type includes three delocalized p MOs(the top three in Figure 6a), which are two degenerate highestoccupied molecular orbitals (HOMOs, 1e’’) and the HOMO-3(1a2’’). The three p MOs involve six p electrons and give p aro-maticity to the N3

3� ring according to the famous 4n+2 elec-tron-counting rule.The second type includes three delocalized sp MOs formed

by in-plane ps orbitals (the middle three in Figure 6a). The twodegenerate HOMO-1 (2e’) and the HOMO-2 (2a1’) have sixsp electrons in total and are expected to render sp aromaticity.The third type includes three delocalized ss MOs formed by

the valence s atomic orbitals (AOs, the bottom three in Fig-ure 6a). The two degenerate HOMO-4 (1e’) and the HOMO-5(1a1’) involve six ss electrons and are expected to render ss aro-maticity. Thus, the N3

3� ring should be considered as beingthreefold aromatic, namely, p, sp, and ss aromatic.MO analysis of the five MN3M (M=Be, B, Mg, Al, and Ca)

species indicates that the BeN3Be, MgN3Mg, and CaN3Ca spe-cies have similar valence MO pictures, and AlN3Al has similarMO pictures to those of BN3B, so we only show the valenceMOs of two representative BeN3Be and BN3B species (see Fig-ures 6b and 6c). As the five MN3M species are open-shell radi-cals, the HOMOs of these species are all singly occupied.From Figure 6b, the 3a1’-HOMO of BeN3Be is a sum of the

s AOs of the two Be atoms, and the remaining nine MOs fromthe N3 subunit are similar to the set of the occupied MOs ofthe N3

3� ring. From Figure 6c, the 2a2’’-HOMO and the 3a1’-HOMO-1 of BN3B are both the sum of the s AOs of the twoB atoms with little s–p mixing. By comparing Figures 6c and6a, we also see clearly that the occupied MOs from the N3 sub-unit in BN3B are similar to those of the N3

3� ring. On the basisof these MO analyses, a p-delocalized system and two s-delo-calized systems contribute to the threefold aromaticity of the

N33� ring and the five MN3M species due to the presence of six

electrons in each delocalized system, which follows the 4n+2electron-counting rule.

Figure 6. Molecular orbital pictures of the bare N33� anion, D3h BeN3Be, and

BN3B. a) The HOMO down to the fifth valence molecular orbital (HOMO-5)for N3

3� ; b) the HOMO down to the sixth valence molecular orbital (HOMO-6) for BeN3Be; c) the HOMO down to the seventh valence molecular orbital(HOMO-7) for BN3B.

Figure 5. Relationship between the NICS values and the distance (d) fromthe center of the N3 ring for the isolated N3

3� trianion and the N3 subunit inBN3B. The NICS values are in ppm and d in I.




The natural bond orbital (NBO) charges and Wiberg bond in-dices (WBI) for the five MN3M species at the MP2/6-311+G(3d)level are listed in Table 3. The NBO analysis indicates that the

BeN3Be, MgN3Mg, and CaN3Ca species all have two M cationsand a �3 charged N3 ring in the middle of the species. TheNBO charges on the N3 subunit are �2.818 in MgN3Mg, �2.726in CaN3Ca, and �2.716 in BeN3Be, and the correspondingcharges on the M atom are Q(Mg)=+1.409, Q(Ca)=+1.363,and Q(Be)=+1.358. Thus, the three MN3M (M=Be, Mg, Ca)species can be viewed formally as complexes of the N3

3� trian-ion and two metal cations. Consequently, the aromaticity ofthese three MN3M species is from the N3

3� ring in the middleof the species.The BN3B and AlN3Al species still exhibit aromaticity based

on the regular triangular geometry of the N3 subunit, negativeNICS values, and MO analyses, although the NBO charges onthe N3 subunit are relatively small at �1.264 in BN3B and�2.308 in AlN3Al. The WBI are larger at 0.492 for the B�Nbond and 0.213 for the Al�N bond, which indicates some co-valent character between the N3 ring and B (or Al). This resultsin some electron sharing between the N3 ring and B (or Al) tomaintain the aromaticity of the N3 ring in the BN3B and AlN3Alspecies.

2.3. Stability and VIE of the MN3M Species

The zero-point-corrected MP2 energies DE for two hypotheticalreactions MN3M!2M+N3 (DE1) and MN3M!2M+3N (DE2) areshown in Table 4. The two reactions are endothermic accord-ing to the positive DE values, which indicates that the five

MN3M species are stable toward decomposition. So it is rea-sonably believed that the five MN3M species could possiblyexist. Furthermore, the order of DE values is MgN3Mg<CaN3Ca<BeN3Be<AlN3Al<BN3B. The DE value increases withincreasing WBI value of the M�N bond. For instance, the BN3Band AlN3Al species possess large DE values, which result fromthe relatively larger covalent character between the N3 ringand B (or Al).The vertical ionization energies (VIEs) of the MN3M (M=Be,

B, Mg, Al, Ca) species obtained by using five methods are pre-sented in Table 5. For all the species, the VIE value from the

MP2 method is very near to that from the CCSD(T) method.The order of the VIE values is 8.15 (BN3B)>7.27 (AlN3Al)>6.23(BeN3Be)>5.47 (MgN3Mg)>3.64 eV (CaN3Ca) at the CCSD(T)/6-311+G(3d) level. We found that the VIE values of the MN3Mspecies increase with the second ionization energy (IE)[37] ofthe M atom (see Figure 7). A possible explanation is given asfollows. The HOMO of the MN3M species almost comes fromthe two M atoms. The electron removed from the MN3M spe-cies is actually removed from the M atom in the ionization pro-cess. Since the M atom in the MN3M species is not really a neu-tral atom but a cation, the magnitude of the VIE of the MN3Mspecies does not correlate with the first IE of the M atom butcorrelates with the second IE.The [CCSD(T)-SCF]/CCSD(T) component represents the elec-

tron correlation contribution. In comparison to the CCSD(T)method (Table 5), the SCF method underestimates the VIE of

the BeN3Be (11.4%), MgN3Mg(9.2%), and CaN3Ca (11.6%) spe-cies, whereas it overestimatesthe VIE of the BN3B (�24.9%)and AlN3Al (�18.4%) species. Forthe three MN3M (M=Be, Mg,and Ca) species with obviousionic character between theM atom and N3 subunit, the elec-tron correlation contribution ispositive and increases the VIE.For the BN3B and AlN3Al specieswith some covalent character

Table 3. Natural bond orbital (NBO) charges and Wiberg bond indices(WBI) for the MN3M (M=Be, B, Mg, Al, Ca) species at the MP2/6-311+G(3d) level.

Species MgN3Mg CaN3Ca BeN3Be AlN3Al BN3B

chargesM 1.409 1.363 1.358 1.154 0.632N3 ring �2.818 �2.726 �2.716 �2.308 �1.264WBIN···N 1.002 1.006 1.006 0.988 0.921M···N 0.076 0.096 0.102 0.213 0.492M···M 0.206 0.221 0.223 0.205 0.236

Table 4. Total energies [E(MN3M)] and zero-point energy (ZPE) for the MN3M (M=Be, B, Mg, Al, Ca) species, en-ergies for the M atoms [E(M)] at the MP2/6-311+G(3d) level, and zero-point-corrected MP2 energies DE1 andDE2 for the hypothetical MN3M!2M+N3

[a] and MN3M!2M+3N reactions, respectively.

Species E(MN3M) [Eh] ZPE [eV] E(M) [Eh] DE1 [eV] DE2 [eV]

MgN3Mg �563.057173 0.32 �199.6289178 1.16 8.98CaN3Ca �1517.829131 0.31 �676.9676028 3.74 11.56BeN3Be �193.134476 0.44 �14.5989837 4.77 12.59AlN3Al �647.777708 0.35 �241.9069418 5.60 13.42BN3B �213.181559 0.46 �24.572281 7.48 15.31

[a] The zero-point-corrected MP2 energy for the N3 radical is �163.7450701 a.u.

Table 5. VIE values [eV] for the MN3M (M=Be, B, Mg, Al, Ca) species atthe CCSD(T)/6-311+G(3d) level.

CaN3Ca MgN3Mg BeN3Be AlN3Al BN3B

SCF 3.22 4.97 5.52 8.61 10.18MP2 3.54 5.36 6.16 7.18 8.07MP4(SDQ) 3.56 5.36 6.11 7.46 8.62CCSD 3.57 5.37 6.12 7.58 8.68CCSD(T) 3.64 5.47 6.23 7.27 8.15CCSD(T)-SCF 0.42 0.50 0.71 �1.34 �2.03[CCSD(T)-SCF]/CCSD(T)

11.6% 9.2% 11.4% �18.4% �24.9%

the second IE of M(in eV)

11.87 15.04 18.21 18.83 25.15


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between the M atom and N3 subunit, the electron correlationcontribution is negative and decreases the VIE. The electroncorrelation contribution of the VIE in the latter two species islarger than those in the former three species. This findingshows that the magnitude and sign (positive or negative) ofthe electron correlation contribution is related to the characterof bonding between the M atom and the N3 subunit in theMN3M species.From the data above, the electron correlation contribution is

important for calculating the VIE values of the MN3M species.As a result, it is necessary to calculate the VIE by using a high-level method including electron correlation.

2.4. Superalkali Characteristics of the CaN3Ca Species

Alkali-metal atoms possess the lowest ionization energiesamong all the elements, and the Cs atom possesses the lowestIE of 3.90 eV[37] among the alkali metals. There is a class of mol-ecules, known as superalkalis[43] , which possess lower IEs thanalkali-metal atoms. Molecules with low IEs may play an impor-tant role in chemistry. The low IE of superalkali species can beused in the synthesis of a new class of charge-transfer salts inwhich the corresponding anions are formed by the specieswith low electron affinity. The lowest experimentally measuredelectron detachment energy was found for Li3O: 3.54�0.30 eV.[44] In the MN3M species, the lowest VIE value is 3.64 eVfor CaN3Ca, which is even lower than the IE=3.90 eV of the Csatom. Thus, CaN3Ca can be considered as a superalkali species.As mentioned in Section 2.3, the magnitude of the VIE of

the MN3M species correlates with the second IE of the M atom.However, the second IE of the Ca atom is 11.87 eV (seeTable 5), so why does the CaN3Ca species have a VIE value aslow as 3.64 eV? Here, the repulsive interaction between theN3

3� trianion and the s-electron clouds of two Ca cations playsan important role.To show the influence of the repulsive interaction, a Ca2

3+

model is constructed (keeping the two Ca cations fixed but re-moving the N3

3� trianion). As illustrated in Figure 8, withoutN3

3� being sandwiched the s-electron clouds are around thetwo Ca cations in the Ca2

3+ model. The orbital energies of the

Ca23+ model are �0.76 a.u. for the HOMO and �0.46 a.u. for

the LUMO. In addition, the VIE of the Ca23+ model is 20.89 eV

at the CCSD(T)/6-311+G(3d) level. Interestingly, after the N33�

trianion is sandwiched, the s-electron clouds of the two Ca cat-ions are clearly distended away from the Ca cations and theCaN3Ca species exhibits much higher orbital energies of�0.12 a.u. for the HOMO and �0.05 a.u. for the LUMO. Consid-ering the distending electron clouds and high orbital energy ofthe HOMO, it is reasonable for the CaN3Ca species to be proneto losing an electron and consequently have a low VIE value.The existence of superalkalis suggests that it should be pos-

sible to create solid salts by reaction with electronegative spe-cies such as halogens. To further identify this superalkali char-acteristic of the CaN3Ca species, the geometry of its chloride isoptimized. A steady C3v structure of the CaN3CaCl molecule(see Figure 9) with all real frequencies is obtained at the MP2/6-311+G(3d) level. The CaN3CaCl molecule corresponds to analmost-perfect trigonal bipyramidal CaN3Ca moiety with the Clatom occupying an on-top site (linking with one of the Caatoms). Interestingly, the structure of the CaN3Ca moiety is

Figure 7. Behavior of the VIE with increasing second ionization energy of M.

Figure 8. The N33� trianion repulses the s-electron clouds of two Ca cations,

which results in a HOMO with distended electron clouds and a high energyfor the CaN3Ca species.

Figure 9. Optimized geometry of the CaN3CaCl molecule.




almost unaffected by the introduction of the Cl atom. Aftercomplexing with the Cl atom, the Ca···N distance only changesby 0.07 I, and the N�N bond lengthens by 0.007 I.The atoms-in-molecule (AIM) approach was employed to

characterize the Ca�Cl bond in the CaN3CaCl molecule. TheLaplacian of the electron density at a bond critical point,521(r), is calculated at the MP2/6-311+G(3d) level. Popeli-er[45,46] proposed that for covalent bonds the value of 521(r) isnegative. For ionic bonds, hydrogen bonds, and van der Waalsinteractions the value of 521(r) is positive. The 521(r) value ofthe Ca�Cl bond is 0.1759 a.u. , which indicates the ionic charac-ter of this bond in the CaN3CaCl molecule. The NBO analysisshows that the charges are �0.946 on the Cl atom and+0.946 on the CaN3Ca moiety, which suggests an electrontransfer from CaN3Ca to the Cl atom in forming the CaN3CaClmolecule. The charge on the N3 ring shows little change; how-ever, there is a charge increase of +0.944 e on the two Caatoms, which act as electron donors, and the Cl atom acceptsan electron to become the Cl� anion. The additional positivecharge on the CaN3Ca moiety was originally imagined to local-ize on the Ca atom linked to the Cl atom. However, this is notthe case, as the two Ca atoms seem to share a similar addition-al positive charge during complexation with the Cl atom.Comparisons between the MOs of CaN3CaCl and those of

NaCl are shown in Figure 10. Interestingly, the MOs of CaN3-CaCl appear to resemble those of NaCl, only the MO orderingis different.

In summary, in the CaN3CaCl molecule, the structure of theCaN3Ca moiety is almost the same as that of the isolatedCaN3Ca species. In the CaN3Ca moiety, the two Ca atoms sharealike the additional positive charge; the MOs of the CaN3CaCl

molecule resemble those of a typical alkali halide (NaCl). Un-doubtedly, the CaN3Ca species exhibits the characteristics of asuperalkali atom.

3. Conclusions

We have investigated a new aromatic N33� ring and a series of

trigonal bipyramidal MN3M (M=Be, B, Mg, Al, Ca) species, forwhich no report has been found so far. The N3

3� ring and theN3 subunit in the MN3M species both exhibit regular triangulargeometries. The NICS values are negative for the N3

3� ring andthe five MN3M species. From MO analysis, it is known that theN3

3� ring has three delocalized p, three delocalized sp, andthree delocalized ss MOs. Each of the delocalized chemical-bonding systems satisfies the 4n+2 electron-counting ruleand therefore exhibits the characteristics of aromaticity. Simi-larly, the five MN3M species and the N3

3� ring have the sameset of MOs and exhibit threefold aromaticity.From NBO analysis, the three MN3M (M=Be, Mg, Ca) species

contain the N33� ring and an ionic bond between the M atom

and N3 subunit. Their DE and VIE values are relatively small.For the BN3B and AlN3Al species with some covalent characterbetween the M atom and N3 subunit, the DE and VIE valuesare relatively larger compared to those of the former threespecies.As a result of the repulsion between the N3

3� trianion andthe electron clouds of the two Ca cations, the CaN3Ca specieshas a very low VIE value of 3.64 eV, which is even lower thanthat of the Cs atom. A further study on the structure of theCaN3CaCl molecule has confirmed the superalkali characteris-tics of CaN3Ca. In the CaN3CaCl molecule, the CaN3Ca moietyretains the geometry of the isolated CaN3Ca species. In the[CaN3Ca]

+ moiety, the two Ca atoms share alike the additionalpositive charges; the MOs of the CaN3CaCl molecule resemblethose of a typical alkali halide (NaCl). Therefore, the CaN3Caspecies is a new superalkali atom. In addition, the characteristicvibrational frequencies calculated for the MN3M species maybe useful in the experimental identification of the MN3M (M=

Be, B, Mg, Al, Ca) species.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (No.20573043, 20503010).

Keywords: ab initio calculations · aromaticity · clustercompounds · nitrogen · superalkali species

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Received: May 27, 2005Revised: July 30, 2005Published online on November 14, 2005




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The Trigonal Bipyramidal MN3M Species: A New Kind of Aromatic Complex Containing a Multiple-Fold Aromatic N3 Subunit