Probing the Electronic Structures and Properties of Neutral and

Article pubs.acs.org/JPCA

Probing the Electronic Structures and Properties of Neutral and Charged Monomethylated Arsenic Species (CH3Asn(−1,0,+1), n = 1−7) Using Gaussian‑3 Theory Xue Bai,† Qiancheng Zhang,† Jucai Yang,*,‡ and Hongmei Ning† †

School of Chemical Engineering, Inner Mongolia University of Technology, Hohhot, 010051, P.R. China School of Energy and Power Engineering, Inner Mongolia University of Technology, Hohhot, 010051, P.R. China

‡

ABSTRACT: The structures and energies of neutral and charged monomethylated arsenic species CH3Asn(−1,0,+1) (n = 1−7) have been systematically investigated with the Gaussian-3 (G3) method. The ground-state structures of monomethylated arsenic species including the neutrals and the ions are vertexmethylated type. The lowest-energy structures of neutral methylated arsenic species and their ions can be viewed as being derived from corresponding to neutral and ionic arsenic clusters, respectively. The reliable electron affinities and ionization potentials of CH3Asn have been evaluated. And there are odd−even alternations in both electron affinities and ionization potentials as a function of size of CH3Asn. The dissociation energies of CH3 from neutral CH3Asn and their ions have been calculated to examine relative stabilities. The results characterized the odd-numbered neutral CH3Asn as more stable than the even-numbered systems, and the even-numbered cationic CH3Asn+ as more stable than the oddnumbered species with the exception of n = 1. The dissociation energy of CH3As+ is the maximum among all of these values. There are no odd−even alternations for anionic CH3Asn− with n ≤ 7.

1. INTRODUCTION Over the past two decades, small arsenic clusters have been studied both experimentally and theoretically because of their applications in semiconductors, optoelectronics, and biopharmaceutics, and their intrinsic interest from the point of view of chemical structure and bonding.1−4 For Asn clusters, the ground-state structures confirmed by theoretical methods3−6 and experimental schemes, such as photoelectron spectroscopy,1,7−10 Raman measurements,11 or gas-phase electron diffraction analysis,2 are line for As2, isosceles triangle for As3, tetrahedral geometry for As4, and distorted trigonal bipyramid for As5. The predicted lowest-energy ground states for n ≥ 6 were found to be dependent on the type of calculation and also on the optimization technique. Many calculations4,5,12,13 showed that the type of benzvalene form and type of trigonal prism compete with each other for the ground-state structures of As6 and As7, respectively. Although methylated arsenic compounds are less studied with quantum chemistry methods now, methylated arsenic species are expected to attract much attention because they can be not only used as a source of arsenic in microelectronics industry and a building block to other organoarsenic species, but are also involved in biogeochemical cycle of arsenic.14−17 Methylated arsenic compounds play key roles in chemical vapor deposition of thin films and in environmental behavior of arsenic. Lide Jr.18 analyzed the rotational spectrum of the ground vibrational state of (CH3)3As using a microwave spectrum. Elbel et al.19 measured the ionization potential (IP) of (CH3)3As with photoelectron spectrometry. Here we have © 2012 American Chemical Society

carried out a detailed study of the neutral and charged methylated arsenic species by means of the higher level of the Gaussian-3 (G3) theory,20,21 intending to provide the first theoretical description of the structures, electron affinities (EAs), IPs, and dissociation energies (DEs) as a reference for further experimental studies. The G3 theory provides accurate energies of molecular systems for calculation of enthalpies of formation, proton affinities, atomization energies, IPs, and EAs. The average absolute deviations from experiment for the 423 reactions are 1.06 kcal/mol.21 Recently, we calculated the EAs, IPs, and DEs of Asn clusters using this G3 theory. The average absolute deviation from experiment for nine reactions was only 0.05 eV.4

2. COMPUTATIONAL METHODS All of calculations were been performed using the extension of G3 theory20,21 and the Gaussian 03 package.22 The G3 theory is a composite method in which the geometry optimization is performed at the MP2(full)/6-31G(d) level. The energies, a series of single-point energy calculations at the levels of QCISD(T)/6-31G(d), MP4/6-31G(d), MP4/6-31+G(d), MP4/6-31G(2df,p), and MP2(full)/GTlarge are carried out. Then these energies are modified by a series of corrections. Finally, the HF/6-31G(d) vibrational frequencies, scaled by Received: June 9, 2012 Revised: August 28, 2012 Published: August 30, 2012 9382

dx.doi.org/10.1021/jp3056569 | J. Phys. Chem. A 2012, 116, 9382−9390

The Journal of Physical Chemistry A

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than the C3v symmetry of the 4A1 state (not shown in Figure 1) by 1.35 eV. Compared with neutral geometry, the As−C bonds of 1d are shortened by 0.086 Å, and the C−H bonds are elongated by 0.002−0.019 Å. 3.2. CH3As2 and Its Charged Molecules. The equilibrium geometries of the CH3As2 compounds and their charged species are given in Figure 2. Similarly to As2F compound,29 the

0.893, are applied for the zero-point vibrational energy (ZPVE) correction at 0 K. The EAs and IPs of CH3Asn with n = 1−4 and some of the most vague data have also been calculated with the G4 method23 and the Gaussian 09 package24 to check the reliability of the G3 results. In the geometry optimization process, two types of initial geometries for CH3Asn (n = 1−7) and their charged species were taken into account: one type where the ground-state structures of Asn and its charged clusters are somewhat preserved, and another where one or several As−As bonds are broken. Within the first type, three possibilities of vertex-, side-, and face-methylated were considered. Also, within the second type, two possibilities were considered: In one possibility, a CH3 group is located in As−As breaking bonds, and in the other it is located out of the breaking bonds. The stabilities of all of these stationary point geometries have been interrogated by the evaluation of their harmonic vibrational frequencies. Once an imaginary vibrational mode is found, a relaxation along coordinates of imaginary vibrational mode is performed until the true local minimum is actually obtained. In addition, For CH3Asn and its charged species with n = 3, the spin multiplicities of singlet, doublet, triplet, and quartet state were taken into account.

Figure 2. Geometry of CH3As2 and its charged species obtained at the MP2(full)/6-31G(d) level. Only arsenic atoms are numbered. The bond lengths are in angstroms, and bond angles are in degrees.

lowest-energy structures of neutral CH3As2 are Cs symmetry with 2A′ ground state (denoted 2a). The bond lengths are predicted to be 1.990 Å for As−C bonds and 1.090−1.092 Å for three C−H bonds, and the bond angles are predicted to be 101.1° for As2−As1−C with the MP2(full)/6-31G(d) scheme. For negatively charged ion CH3As2−, the ground state structure also possesses Cs symmetry, but 1A′ electronic state (denoted 2b). The bond lengths of the 2b structure are longer by 0.031 Å for As−C and 0.002−0.004 Å for C−H than their neutral counterparts. The As−As bond distance is 0.015 Å shorter than that of neutral, while the As2−As1−C bond angles are larger by 5°. The reason is that the additional electron goes into the a′ orbital corresponding to the singly occupied molecular orbitial (SOMO) of the neutral, which is antibonding between the As atom and the CH3 functional group, causing them to move apart. However, the structure of As−As−C is a connected form, making the As−C bond slightly weaker and the As−As bond slightly stronger. For positively charged ion CH3As2+, the ground-state structure has C3v symmetry with 1A1 electronic state (denoted 2c). The bond lengths of As−As and As−C are respectively predicted to be 2.131 and 1.960 Å, which are respectively shorter by 0.119 and 0.030 Å than their neutral counterparts. The three equivalent C−H bond distances are calculated to be 1.091 Å. The As2−As1−C bond angle changes from 101.1° in neutral to 180° in the anion, and the symmetries change from Cs to C3v. 3.3. CH3As3 and Its Charged Molecules. The equilibrium geometries of the CH3As3 and its charged species are exhibited in Figure 3. For neutral CH3As3, three isomers are reported. Structures 3(a) and 3(b) can be viewed as tilted attaching one methyl group to the vertex of the isosceles triangle of the As3 ground-state structure. Both of them display Cs symmetry with 1 A′ electronic state. The isomer 3(b) is a saddle point due to having an imaginary a″ frequency of 29i cm−1 at the MP2(full)/ 6-31G(d) level. Following the mode a″, the isomer 3(b) collapses to the structure 3(a). Energetically, the 3(a) structure is only more stable than the 3(b) by 0.01 eV. That is, the methyl group rotation in energy is negligible. The isomer 3(c), with approximately polyline geometry, also has Cs symmetry with 1A′ electronic state. Energetically, the 3(c) structure is less

3. RESULTS AND DISCUSSION 3.1. CH3As and Its Charged Molecules. The optimized geometries of the CH3As radical and its charged species are shown in Figure 1. Similarly to AsH,25 AsSi+,26 AsF,27 and

Figure 1. Geometry of CH3As and its charged species optimized at the MP2(full)/6-31G(d) level. The bond lengths are in angstroms.

AsCl28 species, the ground state of CH3As is a triplet. The symmetry of the 3A1 ground-state structure of CH3As (denoted 1a) is C3v. At the MP2(full)/6-31G(d) level of theory, the bond length is predicted to be 1.973 Å for As−C bonds and 1.092 Å for three equivalent C−H bonds. The 1b isomer has Cs symmetry with 1A′ state. Energetically, it is higher than the 1a structure by 1.07 eV at the G3 level of theory. The As−C bond length in 1b geometry is shorter than that in the 1a structure by 0.03 Å. A quintet state geometry with C3v symmetry was also calculated, but it was not shown in this paper because it was predicted to lie 2.36 eV above the triplet ground state. For anion CH3As−, both the doublet and quartet electronic states are considered. The ground-state structure (denoted 1c) has Cs symmetry with 2A′ state, which is more stable than that of the quartet by 2.06 eV in energy. Compared with neutral geometry, the bond lengths of 1c are elongated by 0.053 Å for As−C and by 0.004−0.005 Å for C−H bonds. For cation CH3As+, we also considered two electronic states: one is doublet, and the other is quartet. Energetically, the Cs symmetry of the 2A″ ground state (denoted 1d) is more stable 9383



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account of having an imaginary a″ frequency of 68i cm−1 at the MP2(full)/6-31G(d) level. Energetically, the 3(h) structure is less stable than the 3(g) by 0.01 eV. That is, the methyl group rotation in the cation is energetically negligible. The polyline structure 3(i) possesses Cs symmetry with 2A″ electronic state. Energetically, the 3(i) structure is less stable than the 3(g) by 0.66 eV. The two equivalent As−As bond lengths for the 3(g) structure are evaluated to be 2.365 Å, which is 0.094 Å shorter than their neutral counterparts, and 2.336 Å, which is 0.063 Å longer than their neutral counterparts. The As−C and C−H bond distances are calculated to be 1.973 Å and 1.090−1.092 Å, respectively. They are respectively shortened by 0.011 Å and 0−0.003 Å compared with their neutral counterparts. The As3−As1−As2 bond angles are calculated to be 58.3°, which is 4.2° larger than that of the neutral. 3.4. CH3As4 and Its Charged Molecules. The structures of the CH3As4 and its charged species are displayed in Figure 4. When a CH3 group is attached to the vertex of a tetrahedron of As4 ground-state structure,1,4 a vertex-methylated isomer is obtained. However, vibrational analysis yields more than one imaginary frequency, indicating distortion to lower energetic geometry. The vertex-methylated isomer undergoes Jahn− Teller distortion to give an “As−As bond breaking” structure, in which the CH3 group is located out of the breaking bonds, as can been seen from Figure 4a. If a CH3 group is added to a face of the As4 tetrahedron, a face-methylated isomer is obtained. Nevertheless, checking vibrational frequencies, more than one imaginary frequency is found. The face-methylated form suffers Jahn−Teller relaxation to also give an “As−As bond breaking” structure, in which the CH3 group is located in the breaking bonds as can been seen from Figure 4c. Both 4(a) and 4(c) structures display Cs symmetry with 2A′ state. Energetically, the

stable than 3(a) by 1.69 eV. The two equivalent As−As bond lengths for the 3(a) structure are predicted to be 2.459 Å, which is only 0.034 Å longer than that of the As3 ground-state structure, and 2.273 Å, which is only 0.010 Å shorter than that of As3 ground-state geometry. The As−C and C−H bond distances are evaluated to be 1.984 Å and 1.092−1.093 Å, respectively. The As3−As1−As2 bond angles are calculated to be 55.0°, which are only 1.2° smaller than that of the vertex angle of the As3 ground-state structure. For the negatively charged ion CH3As3−, three isomers are presented. The isomers 3(d) and 3(e) display Cs symmetry with 2A″ electronic state. The isomer 3(e) is a saddle point because it has an imaginary a″ frequency of 122i cm−1 at the MP2(full)/6-31G(d) level. Following the mode a″, the isomer 3(e) collapses to the structure 3(d). Energetically, the 3(d) structure is more stable than 3(e) by 0.04 eV. That is, the methyl group rotation in the anion may be energetically negligible. The polyline structure 3(f) has Cs symmetry with 2 A′ electronic state. Energetically, the 3(f) structure is less stable than the 3(d) by 0.78 eV. The two equivalent As−As bond lengths for the structure 3(d) are predicted to be 2.433 Å, which is 0.026 Å shorter than their neutral counterparts, and 2.372 Å, which is longer than that of the neutral by 0.099 Å. The As−C and C−H bond distances are evaluated to be 2.001 Å and 1.092−1.095 Å, respectively. They are respectively elongated by 0.017 Å and 0−0.002 Å compared with their neutral counterparts. The As3−As1−As2 bond angles are calculated to be 58.3°, which is 3.3° larger than their neutral counterparts. For the positively charged ion CH3As3+, three isomers are obtained. The isomers 3(g) and 3(h) have Cs symmetry with 2 A′ electronic state. The isomer 3(h) is a saddle point on 9384



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equivalent bond distances unconnected to the methyl are evaluated to be 2.425 Å, and another is calculated to be 2.460 Å. The C−As and C−H bond distances are respectively evaluated to be 1.989 Å and 1.091−1.093 Å. For anion CH3As4−, five isomers are reported. Both 4(f) and 4(h) structures can be regarded as being derived from the ground-state structure of As4− by adsorbing a methyl group. Both of them possess Cs symmetry with 1A′ electronic state. They are very close in energy. The 4(f) structure is only more stable than the 4(h) by 0.06 eV at the G3 and the G4 levels. The Cs symmetry isomer 4(g) of the 1A′ state is a saddle point due to having an imaginary a″ frequency of 119i cm−1 at the MP2(full)/6-31G(d) level. Energetically, the 4(g) structure is less stable than 4(f) by 0.04 eV. That is, the methyl group rotation may be energetically negligible. The isomer 4(i), corresponding to neutral 4(d), possesses Cs symmetry with 1A′ electronic state. It is energetically less stable than 4(f) by 0.33 eV. The chain structure 4(j) displays Cs symmetry with 1A′ electronic state. Checking vibrational frequencies, it has an imaginary a″ frequency of 25i cm−1 at the MP2(full)/6-31G(d) level. Following the mode a″, it collapses to the 4(i) isomer. Compared with neutral 4(a) structure, the four pairs of equal As−As bond lengths of the 4(f) structure are elongated by 0.01 and 0.015 Å, respectively. Another As−As bond distance is shortened by 0.055 Å. The C−As and C−H bond lengths are elongated by 0.027 and 0.001−0.002 Å, respectively. For cation CH3As4+, five isomers are also reported. Both 4(k) and 4(m) structures can be regarded as being derived from the ground-state structure of As4+ by adsorbing a CH3. They have Cs symmetry with 1A′ electronic state. Energetically, the 4(k) structure is more stable than 4(m) by 0.23 eV. That is, the lowest-energy structure of cation CH3As4+, in which the methyl group is located in the As−As breaking bond, is different from that of neutral and anion, in which the methyl group is located out of the As−As breaking bond. However, it is similar to the ground-state structure of HAs4+, which is the Hbridged (As−H−As) form.30 The Cs symmetry isomer 4(l) of 1 A′ state is a saddle point because it has an imaginary a″ frequency of 152i cm−1 at the MP2(full)/6-31G(d) level. Energetically, the 4(l) structure is less stable than 4(k) by 0.06 eV. The Cs symmetry isomer 4(n) of 1A′ state is a second-order saddle point in the potential energy surface because two imaginary a″ frequencies of 97i and 59i cm−1 at the MP2(full)/ 6-31G(d) level are found. Following the first mode a″, it collapses finally to the isomer 4(m). The polyline structure 4(o) displays Cs symmetry with 1A′ electronic state. It is energetically less stable than the 4(k) by 1.74 eV. For five As− As bonds of the 4(k) structure, two equivalent bond lengths connected to the methyl are predicted to be 2.432 Å, two equivalent bond distances unconnected to the methyl are evaluated to be 2.390 Å, and another is calculated to be 2.580 Å. The C−As and C−H bond distances are evaluated to be 2.021 and 1.089−1.091 Å, respectively. From the discussion above, we can see that the polyline structures of neutral and charged CH3Asn are higher in energy. Hence, the polyline structures of neutral and charged CH3Asn with n ≥ 5 are not considered. 3.5. CH3As5 and Its Charged Molecules. The structures of CH3As5 and its charged species are given in Figure 5. For neutral, three isomers are reported. Both isomers 5(a) and 5(c) are vertex-methylated structure obtained by respectively adding a CH3 group to a horizontal As atom and a vertex As atom of deformed trigonal bipyramid of As5 ground-state structure.1,4


4(a) structure is more stable than the 4(c) by 0.13 eV. The Cs symmetry isomer 4(b) of the 2A′ state is a saddle point because an imaginary a″ frequency of 104i cm−1 at the MP2(full)/631G(d) level is found. Energetically, the 4(b) structure is less stable than 4(a) by 0.03 eV. That is, the methyl group rotation in energy may be negligible. The isomer 4(d) can be regarded as being derived from the ground state structure As3 by adsorbing CH3As. It is Cs symmetry with 2A″ electronic state. Energetically, it is less stable than the structure 4(a) by 0.84 eV. The chain structure 4(e) displays Cs symmetry with the 2A′ electronic state. It is less stable than the 4(a) by 1.42 eV in energy. For the 4(a) ground state structure, among the remaining five As−As bonds, two equivalent bond lengths connected to the methyl are predicted to be 2.457 Å, two 9385



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symmetry changes from Cs to C1. The 5(f) isomer has Cs symmetry with 2A′ electronic state. The coordination number of As atom numbered 1 in the 5(f) isomer is equal to 4, so its energy is very high. For example, it is higher in energy than the 5(d) structure by 1.40 eV. It is clear that the ground state structure of anon CH3As5− differs from that of neutral CH3As5 because the lowest-energy structure of anon As5− differs from that of neutral As5.1,4 The two pairs of equal As−As bond lengths for the ground state 5(d) structure are predicted to be 2.278 and 2.415 Å, respectively. And another is predicted to be 2.313 Å. The As−C and As−H bond distances are evaluated to be 2.000 and 1.091−1.093 Å, respectively. For cation CH3As5+, five isomers are reported. Both 5(g) and 5(h) isomers can be obtained by connecting a methyl group not only to a horizontal As atom of the deformed trigonal bipyramid of the As5 ground-state structure, but also to an As atom of the end of tetragonal pyramid of the As5+ ground-state structure.4,5 The geometries of 5(g) and 5(h) isomers are very similar, and both of them display Cs symmetry, whereas the former is 2A″ ground state and the later is 2A′ electronic state. Energetically, the 5(g) structure is only more stable than the 5(g) by 0.06 eV at the G3 and the G4 level of theory. The 5(j) isomer can be obtained by connecting a methyl group to vertex As atom of tetragonal pyramid of As5+.4,5 It is Cs symmetry with 2 A′ electronic state. It is less stable than the 5(g) structure by 0.63 eV in energy. The 5(i) isomer corresponds to neutral 5(c) structure. It possesses Cs symmetry with 2A″ electronic state. It is higher in energy than the 5(g) structure by 0.27 eV. The 5(k) isomer corresponds to neutral 5(b) structure. It has Cs symmetry with 2A′ electronic state. Energetically, it is less stable than the 5(g) structure by 1.16 eV. The three pairs of equal As−As bond lengths for 5(g) structure are calculated to be 2.434, 2.439, and 2.501 Å, respectively. The As3−As4, As−C, and As−H bond distances are predicted to be 2.528, 1.965, and 1.091−1.094 Å, respectively. The energies of methyl group rotation for the 5(a), 5(d), and 5(g) structures are respectively calculated to be 0.06, 0.08, and 0.03 eV, although the corresponding geometries are not shown in Figure 5. The energies of methyl group rotation are increased with the n increasing. That is, the methyl group rotation may not be energetically negligible when n increases, especially for the anion CH3Asn−. 3.6. CH3As6 and Its Charged Molecules. The structures of CH3As6 and its charged species are shown in Figure 6. The benzvalene form of C2v symmetry and the trigonal prism of D3h symmetry compete with each other for the ground state of the neutral As6 cluster.4 For neutral CH3As6, five isomers are reported. The isomers 6(a) and 6(d) are vertex-methylated structures obtained by attaching a methyl group to an As atom of the benzvalene form of the As6 cluster. The 6(a) structure has C1 symmetry, and it is the most stable in energy. The isomer 6(d) possesses Cs symmetry with 2A″ electronic state. Energetically, it is less stable than 6(a) by 0.74 eV. Isomer 6(c) is obtained by connecting a methyl group to the vertex As atom of the benzvalene form of As6. It has Cs symmetry with 2A″ electronic state and is less stable than 6(a) by 0.49 eV in energy. The isomer 6(b) can be obtained by connecting a methyl group to an As atom of the trigonal prism of the As6 species. The As1−As3 bond distance of 3.401 Å indicates that the isomer 6(a) is an “As−As bond breaking” structure. The isomer 6(b) displays Cs symmetry with 2A′ electronic state. Energetically, it is less stable than that of 6(a) by 0.41 eV. The isomer 6(e) in which the CH3 group is located in the As−As

Figure 5. Geometry of CH3As5 and its charged species obtained at the MP2(full)/6-31G(d) level. Only arsenic atoms are numbered. The bond lengths are in angstroms.

Isomer 5(b) is also a vertex-methylated structure obtained by attaching a CH3 group to a vertex of the regular pentagon of As5− ground-state structure.1,4 All of these display Cs symmetry with 1A′ state. Energetically, the isomers 5(b) and 5(c) are less stable than 5(a) by 0.78 and 1.47 eV, respectively. The three pairs of equal As−As bond lengths for 5(a) structure are predicted to be 2.466, 2.471, and 2.477 Å, respectively. The As3−As4, As−C, and As−H bond distances are evaluated to be 2.442, 1.977, and 1.092−1.093 Å, respectively. Similarly to the neutral, three vertex-methylated isomers for anion CH3As5− are reported. The 5(d) structure, corresponding to neutral 5(b), possesses Cs symmetry with 2A′ ground state. The 5(e) structure has C1 symmetry. Energetically, it is less stable than the 5(d) structure by 0.19 eV. Using the neutral 5(a) as the initial geometry of anion optimization, the Cs symmetry isomer of 2A″ state is obtained. However, vibrational analysis yields an imaginary frequency, indicating distortion to lower symmetry. It undergoes Jahn−Teller distortion to give a C1 symmetry isomer 5(e), in which the As3−As5 bond is broken. In fact, the coordination numbers of As atoms for neutral Asn clusters and their anion are usually less than or equal to 3. In the corresponding neutral 5(a) structure, the coordination numbers of each As atom are equal to 3. So when neutral 5(a) isomer obtains an additional electron and becomes negative ion 5(e), the As3−As5 bond is broken, and the 9386



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optimization. The ground-state structure of CH3As6+, 6(k), possesses Cs symmetry with 1A′ ground state, which is different from that of the neutral CH3As6. Compared with the neutral ground state structure, the shape of the anion primary change is intramolecular CH3-transfer. Isomer 6(l) corresponding to neutral 6(b) has Cs symmetry with 1A′ electronic state. Energetically, it is higher than 6(k) by 0.26 eV. Although Cs symmetry isomer 6(m) of 1A′ state derives from the neutral 6(a), the difference of the geometry of both is very large. The main difference is that the C−As bond in the neutral, approximately perpendicular to the main plane of the As6 benzvalene form, becomes completely parallel with the main plane in the positively charged ion. The 6(m) isomer is less stable than 6(k) by 0.34 eV in energy. Although the geometry of isomer 6(n) differs from that of neutral 6(c), the 6(n) structure derives from the 6(c). The 6(n) structure displays Cs symmetry with 1A′ state. Energetically, it is less stable than the 6(k) by 0.37 eV. When an electron is removed from the neutral 6(e), a positively charged ion with Cs symmetry and 1A′ state is obtained. However, it has an imaginary a″ frequency. Following the mode a″, it collapses to isomer 6(o) with C1 symmetry. Energetically, the 6(o) isomer is less stable than that of 6(k) by 0.91 eV. The eight As−As bond distances for the ground state structure 6(k) are predicted to be 2.247−2.544 Å, and the C− As and C−H bond lengths are calculated to be 1.952 Å and 1.092 Å, respectively. 3.7. CH3As7 and Its Charged Molecules. The structures of the CH3As7 and their charged species are shown in Figure 7. Two types of structural patterns compete with each other for the ground -state structure of neutral As7. One type is derived from the benzvalene form of As6, and the other is derived from the trigonal prism of As6.4 For neutral CH3As7, four isomers are presented. The ground state structure 7(a) can be regarded as being derived from the pattern of the trigonal prism of As7. It has Cs symmetry with 1A′ ground state. The isomers 7(b), 7(c), and 7(d) can be regarded as being derived from the pattern of the benzvalene form of As7. They possess Cs symmetry with 1A′ electronic state. Energetically, they are less stable than 7(a) by 0.05, 0.30, and 0.40 eV, respectively. At the G4 level of theory, the 7(a) structure is energetically more stable than the 7(b) isomer by 0.04 eV. The 10 As−As bond distances for the ground state structure 7(a) are predicted to be 2.402−2.528 Å, and the C−As and C−H bond lengths are calculated to be 1.986 Å and 1.092 Å, respectively. For negatively charged ions CH3As7−, four isomers are reported. The ground state structure 7(e) can be regarded as being derived from not only the ground state structure of As7− by attaching to a CH3 but also the neutral 7(a) by adding an extra electron. When the geometry of neutral (7a) obtains an additional electron, we can obtain the Cs symmetry anionic isomer with 2A′electronic state. Checking vibrational frequencies, it has an imaginary a″ frequency. Following the mode a″, it collapses to isomer 7(e) with C1 symmetry. Isomers 7(f), 7(g), and 7(h), corresponding respectively to neutral 7(c), 7(b), and 7(d), possess Cs symmetry with 2A′ state. They are less stable than 7(e) by 0.38, 0.43, and 0.83 eV in energy, respectively. The nine As−As bond distances for the ground state structure 7(e) are evaluated to be 2.362−2.528 Å, and the C−As and C− H bond lengths are calculated to be 1.990 Å and 1.092−1.093 Å, respectively. For positively charged ions CH3As7+, five isomers are presented. The ground state structure 7(i) can be regarded as being derived from not only the neutral 7(b) by removing an


breaking bond of the trigonal prism of the As6 cluster4 displays Cs symmetry with 2A′ electronic state. It is higher in energy than 6(a) by 0.77 eV. The eight calculated As−As bond distances for the ground state structure 6(a) are 2.406−2.452 Å, and C−As and C−H bond lengths are respectively 1.984 Å and 1.092−1.093 Å. The ground state structure of anion As6− is similar to that of neutral As6.4 Hence, we presented five isomers for anion CH3As6− optimized with the neutral structures mentioned above as the initial geometries of anionic optimization. Similarly to neutral CH3As6, the ground state structure 6(f) of anion CH3As6− has C1 symmetry. Both anion 6(g) and 6(h), corresponding respectively to neutral 6(c) and 6(b), possess Cs symmetry with 1A′ electronic state. Energetically, they are higher than 6(f) by 0.13 and 0.24 eV, respectively. Isomer 6(i) corresponding to neutral 6(e) possesses Cs symmetry with 1A′ electronic state. It is higher in energy than the 6(f) structure by 0.59 eV. Isomer 6(j) corresponding to neutral 6(d) displays C1 symmetry and is less stable than 6(f) by 1.40 eV in energy. The eight As−As bond distances for the ground state structure 6(f) are predicted to be 2.349−2.531 Å, and the C−As and C−H bond lengths are calculated to be 1.989 Å and 1.093−1.095 Å, respectively. The lowest-energy structures of cation As6+ and neutral As6 are alike,4 so we also optimized five isomers with the neutral structures mentioned above as the initial geometry of cationic 9387



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ions are methylated, the As−As bonds of arsenic clusters are sometimes broken. However, the methyl group tends to locate out of the breaking bonds with the exception of positively charged ion CH3As4+. The maximum coordination numbers of arsenic atom connected methyl group can be up to four for positively charged ion monomethylated arsenic species, while for neutrals and anions only up to three. The lowest-energy structures of neutral methylated arsenic species can be viewed as being derived from corresponding neutral arsenic clusters, and their ions can be viewed as being derived from corresponding ionic arsenic clusters. If the geometries of neutral Asn clusters are similar to those of their ions, the geometries of CH3Asn species are also similar to those of their ions. For example, the geometries of ground-state structure of neutrals and their anions without n = 5 of CH3Asn are generally alike, because the geometries of ground state structure of neutral Asn clusters are similar to those of their anion compounds. The lowest energy structures of neutral CH3Asn with n = 6 and 7 are different from those of their cations for the reason mentioned above. 3.8. Electron Affinity and Ionization Potential. The adiabatic electron affinity (AEA) (defined as the difference of total energies in the manner AEA = E(the ground state structure of neutral) − E(the ground state structure of anion)) of CH3Asn species is calculated at the G3 level. The ZPVEcorrected AEAs of CH3Asn are evaluated to be 0.66 eV for CH3As, 1.59 eV for CH3As2, 1.40 eV for CH3As3, 1.86 eV for CH3As4, 0.73 eV for CH3As5, 2.34 eV for CH3As6, and 1.64 eV for CH3As7. (At the G4 level, the ZPVE-corrected AEAs of CH3Asn are evaluated to be 0.76 eV for CH3As, 1.64 eV for CH3As2, 1.50 eV for CH3As3, and 1.90 eV for CH3As4. The G4 result is in agreement with that of the G3 theory). To facilitate comparison, Figure 8 sketched the AEAs of CH3Asn and Asn

Figure 7. Geometry of CH3As7 and its charged species obtained at the MP2(full)/6-31G(d) level. Only arsenic atoms are numbered. The bond lengths are in angstroms.

electron but also the ground-state structure of As7+4 by adsorbing a CH3. The 7(i) structure displays Cs symmetry with 2 A″ ground state. If it possesses 2A′ electronic state (see isomer 7(k)), it is higher in energy than the 2A″ ground state by 0.20 eV. Both isomers 7(j) and 7(l), corresponding respectively to neutral 7(c) and 7(d), possess Cs symmetry with 2A′ electronic state. They are higher in energy than the 7(i) by 0.11 and 0.25 eV, respectively. Isomer 7(m), corresponding to the ground state structure of neutral, displays C1 symmetry. Energetically, it is less stable than that of 7(i) by 0.30 eV. The ten As−As bond lengths for the ground state structure 7(i) are evaluated to be 2.393−2.569 Å, and the C−As and C−H bond lengths are calculated to be 1.977 Å and 1.091−1.092 Å, respectively. From the discussion above we can conclude that the ground state structures of methylated arsenic species are the vertexmethylated type. When arsenic species of neutrals and their

Figure 8. The AEA and the AIP versus the number of atoms n for CH3Asn and Asn species. The AEA and AIP of Asn clusters are taken from ref 3.

species as a function of the size of the compounds at G3 level. From Figure 8 we can conclude that (i) the AEAs of CH3Asn compounds become larger when n is even, and are smaller when n is an odd number. This odd−even alternation may be readily explained. With even n, CH3Asn has an open shell electronic structure. When it obtains an electron, the electronic structure changes to a closed shell, which is more stable. With odd n, CH3Asn with the exception of CH3As has a closed shell 9388



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electronic structure. The situation is the opposite, so the AEAs are smaller. (ii) The AEAs of CH3Asn species are larger than those of Asn when n is even, and are smaller than those of Asn when n is an odd number. The reason as mentioned above is that CH3Asn− is a closed shell electronic structure, while Asn− is an open shell electronic structure when n is even. The situation is the opposite when n is odd. No experimental values for CH3Asn are available for comparison. The adiabatic ionization potential (AIP) (defined as the difference of total energies in the manner AIP = E(the ground state structure of cation) − E(the ground state structure of neutral)) of CH3Asn species are evaluated at the G3 level. The ZPVE-corrected AIPs of CH3Asn are estimated to be 8.81 eV for CH3As, 7.94 eV for CH3As2, 8.43 eV for CH3As3, 7.13 eV for CH3As4, 7.67 eV for CH3As5, 6.89 eV for CH3As6, and 7.80 eV for CH3As7. (At the G4 level, the ZPVE-corrected AIPs of CH3Asn are predicted to be 8.78 eV for CH3As, 7.96 eV for CH3As2, 8.41 eV for CH3As3, 7.19 eV for CH3As4. The G4 results are in excellent agreement with the G3 outcome.) To facilitate comparison, the AIPs of CH3Asn and Asn species as a function of the size of the compounds are also shown in Figure 8. As expected, the result of AIPs is opposite to that of AEAs. There are odd−even alternations in the AIPs as a function of size of CH3Asn with smaller AIPs being associated with oddnumber species. The AIPs of CH3Asn species with the exception of CH3As are larger than those of Asn when n is odd, and are smaller than those of Asn when n is an even number, as can be seen from Figure 8. The reason is that CH3Asn with the exception of CH3As is a closed shell electronic structure when n is an odd number. It is more stable than that of an open shell electronic structure (when n is an even number). That is, for species of open shell electronic structure, it is easier to remove an electron than for those of closed shell electron structure. For Asn, the situation is opposite to CH3Asn. No experimental values for CH 3 As n are available for comparison. To check the reliability of the predicting G3 results, we also calculated the AIP of (CH3)3As and compared it with the experimental value. The results show that the theoretical AIP of 8.09 eV is in good agreement with the experimental value of 8.2 eV.19,31 Our theoretical CH3Asn results may thus provide a reference for further investigations. 3.9. Dissociation Energy. The energies of CH3 functional group dissociating from CH3Asn and their ions (defined as the dissociation energy DE = E(CH 3 ) + E(As n (0/∓) ) − E(CH3Asn(0/∓))) are estimated and listed in Table 1 and plotted in Figure 9, respectively. Earlier works4 have exhibited

Figure 9. DEs of CH3 from CH3Asn and their ions versus the number of atoms n for CH3Asn and their ions.

that the theoretical G3 DEs of Asn clusters are in good agreement with the experimental values. For these DEs, there are no experimental values and other theoretical results for comparison. Therefore, the DEs reported in this article may provide a reference for further investigations. From these DEs, the stability of bonding a CH3 to Asn clusters and their ions can be found. The higher values of these DEs indicate that the bonding of a CH3 is stable. As can be seen from Figure 9, the results characterized the odd-numbered neutral CH3Asn as more stable than the even-numbered systems because the odd-numbered species with the exception of CH3As are closed shell electronic structures. For positively charged ions CH3Asn+, the results characterized the evennumbered cationic CH3Asn+ as more stable than the oddnumbered species with the exception of n = 1. The DE of CH3As+ is the maximum among all of these values. For negatively charged ions CH3Asn−, There are no odd−even alternations in the DEs as a function of size of CH3Asn− with n ≤ 7 because the lowest energy structure of As3− is the triplet ground state, not the singlet state.4 As a result, the DE of CH3As3− is slightly larger than that of its neighboring CH3As2− and CH3As4−. The smaller DE of CH3As5− not only indicates that is it less stable, but also that the anionic As5− is more stable.

4. CONCLUSIONS The structures and energies of neutral and charged monomethylated arsenic species CH3Asn(−1,0,+1) (n = 1−7) have been systematically studied by means of the G3 scheme. The ground state structures of monomethylated arsenic species including neutrals and their ions are the vertex-methylated type. The lowest-energy structures of neutral methylated arsenic species can be viewed as being derived from corresponding neutral arsenic clusters, and their ions can be viewed as being derived from corresponding ionic arsenic clusters. The reliable AEAs of CH3Asn have been evaluated to be 0.66 eV for CH3As, 1.59 eV for CH3As2, 1.40 eV for CH3As3, 1.86 eV for CH3As4, 0.73 eV for CH3As5, 2.34 eV for CH3As6, and 1.64 eV for CH3As7. The reliable AIPs of CH3Asn have been estimated to be 8.81 eV for CH3As, 7.94 eV for CH3As2, 8.43 eV for CH3As3, 7.13 eV for CH3As4, 7.67 eV for CH3As5, 6.89 eV for CH3As6, and 7.80 eV for CH3As7. There are odd−even alternations in both AEAs and AIPs as a function of size of CH3Asn. The DEs of CH3 from neutral CH3Asn and their ions

Table 1. DEs of CH3 from CH3Asn and Their Ionsa

a

dissociation

DE

dissociation

DE

CH3As→CH3+As CH3As2→CH3+As2 CH3As3→CH3+As3 CH3As4→CH3+As4 CH3As5→CH3+As5 CH3As6→CH3+As6 CH3As7→CH3+As7 CH3As+→CH3+As+ CH3As2+→CH3+As2+ CH3As3+→CH3+As3+ CH3As4+→CH3+As4+

2.35 1.03 2.42 0.66 2.51 1.70 2.56 3.34 2.96 1.35 2.18

CH3As−→CH3+As− CH3As2−→CH3+As2− CH3As3−→CH3+As3− CH3As4−→CH3+As4− CH3As5−→CH3+As5− CH3As6−→CH3+As6− CH3As7−→CH3+As7− CH3As5+→CH3+As5+ CH3As6+→CH3+As6+ CH3As7+→CH3+As7+

2.31 1.85 2.10 2.05 0.78 2.06 1.34 1.51 2.76 1.34

The values are corrected with ZPVE and in eV. 9389



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(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (23) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. J. Chem. Phys. 2007, 126, 084108-1−084108-12. (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision C.01; Gaussian, Inc.: Wallingford, CT, 2010. (25) Beutel, M.; Setzer, K. D.; Shestakov, O.; Fink, E. H. J. Mol. Spectrosc. 1996, 178, 165−171. (26) Gao, A.; Li, G.; Chang, Y.; Chen, H.; Li, Q. J. Mol. Struct. (THEOCHEM) 2010, 961, 88−96. (27) Xu, W.; Li, G.; Yu, G.; Zhao, Y.; Li, Q. J. Phys. Chem. A 2003, 107, 258−266. (28) Mok, D. K. W.; Lee, E. P. F.; Chau, F.; Dyke, J. M. Phys. Chem. Chem. Phys. 2011, 13, 9540−9553. (29) Kasalová, V.; Schaefer, H. F. J. Comput. Chem. 2005, 26, 411− 435. (30) Alcamí, M.; Mó, O.; Yáňez, M. J. Chem. Phys. 1998, 108, 8957. (31) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Ion Energetics Data. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, 2005 (http://webbook.nist.gov).

have been calculated to examine relative stabilities. The results characterize the odd-numbered neutral CH3Asn as more stable than the even-numbered systems, and the even-numbered cationic CH3Asn+ as more stable than the odd-numbered species with the exception of n = 1. The DE of CH3As+ is the maximum among all of these values. There are no odd−even alternations for anionic CH3Asn− with n ≤ 7. To our knowledge, there are no experimental data and other theoretical results regarding the AEAs, AIPs, and DEs for CH3Asn system. We hope that our predictions will provide strong motivation for further experimental investigation of these important monomethylated arsenic species.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-471-6576145; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Inner Mongolia Talent Foundation from the Inner Mongolia Department of Human Resources and Social Security and by a grant (Grant No. 2009MS0208) from the Inner Mongolia Natural Science Foundation.

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REFERENCES

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Probing the Electronic Structures and Properties of Neutral and

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