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Energetic Properties and Electronic Structure of [C,N,O,P] and [C,N,S,P] Isomers Brian Finney, Kanchana Sahan Thanthiriwatte, Joseph S. Francisco, and David A Dixon J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12673 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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The Journal of Physical Chemistry

Energetic Properties and Electronic Structure of [C,N,O,P] and [C,N,S,P] Isomers Brian Finney,a K. Sahan Thanthiriwatte, b Joseph S. Francisco, a,c and David A. Dixon b, *,† a

Department of Chemistry, Purdue University, West Lafayette, IN, 47907-2084, USA

b

Department of Chemistry, The University of Alabama, Shelby Hall, Tuscaloosa, Alabama

35487-0336, USA c

Department of Chemistry and Office of the Dean, University of Nebraska-Lincoln, Lincoln,

NE, 68588-0312, USA Abstract Correlated molecular orbital theory at the coupled cluster CCSD(T) level with augmented correlation consistent basis sets including F12 explicit correlation has been used to predict the structure and energetic properties of the isomers of [C,N,O,P] and [C,N,S,P]. The predicted ground states are the species derived from a trivalent P with a P=O or P=S bond and a cyano group bonded to the P. The other low energy isomers are the isonitriles and they are 1.4 kcal/mol and 6.6 less stable than the ground state for P=O and P=S, respectively. An analysis of the bond energies is provided and the values are compared to the corresponding [N,N,C,O] isomers. Data is provided for searching for these species in interstellar regions.

†

Email: [email protected] 1 ACS Paragon Plus Environment


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Introduction The importance of the PO bond in nature has been long known, for example, its role in biomolecules such as ATP and DNA. The PS bond is found much less frequently than the PO bond, but has technical applications as phosphorus sulfides are used to produce “strikeanywhere” matches and thiophosphates are present in insecticides and oil additives. Astrochemically, there has been only one molecule containing the PO bond detected and none containing a PS bond. Furthermore, of the approximately 200 molecules detected in the interstellar medium or circumstellar envelopes, only six containing phosphorus have been identified to date: CP, 1 PN, 2,3 PO, 4 CCP,5 PH3, 6,7,8 and HCP. 9 Other phosphorus bearing molecules may exist, specifically those containing a PO bond or even a PS bond. Two such examples are the OPCN and SPCN molecules, which could be formed through the reaction of either PO or PS with the previously detected HCN molecule. 10,11,12 There is also significant interest in whether such species are cyanides or isocyanides. Both OPCN 13 and SPCN 14 were reported to have been formed by pyrolysis and identified by infrared spectroscopy. The CN stretch of OPCN was assigned at 2165 cm-1 and the OP stretch was assigned at 1385 cm-1. For SPCN, the CN stretch was assigned at 2151 cm-1, the SP stretch at 743 cm-1. However, B3LYP/aug-cc-pVTZ calculations and comparison of the spectra with that of possible side products from the pyrolysis synthesis have led to the suggestion that that these assignments are not correct. 15,16 These latter authors with the aid of density functional theory calculations showed that two of the bands assigned to SPCN area actually those of cyanogen and suggested that low resolution infrared analysis of the products of pyrolysis reactions can lead to incorrect assignments.16 They provided a similar analysis of the data for OPCN and reached the conclusion that the molecule had not been observed.15 The binding enthalpy of OPCN has also

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been theoretically studied with a Mg2+ water complex at the B3LYP/6-31+G(d) level. 17 The conflicting information and lack of high level, detailed studies regarding OPCN and SPCN clearly indicates the need for a complete analysis of the entire set of [C,N,O,P] and [C,N,S,P] isomers using higher level computational methods. We have conducted an extensive high level study at the CCSD(T) and CCSD(T)-F12 level of the structures and vibrational spectra of the [C,N,O,P] and [C,N,S,P] isomers. The Feller-Peterson-Dixon approach 18,19,20,21 was then used to predict the heats of formation of the two lowest energy isomers in each set. Computational Approaches The CCSD(T) calculations 22,23,24,25 were done with the correlation consistent aug-ccpVnZ (n= D, T, Q, 5) basis sets 26 for C, N, and O and the aug-cc-pV(n+d)Z basis sets (n= D, T, Q, 5) including tight d functions 27 for the second row S and P atoms. These basis sets are denoted aVnZ. Geometry optimizations and vibrational frequencies were performed at the CCSD(T) level. The electronic structures of the open-shell species were calculated with the R/UCCSD(T) approach where a restricted open shell Hartree-Fock (ROHF) calculation was initially performed and the spin constraint was then relaxed in the coupled cluster calculation. 28,29 This method has been found to give excellent agreement with experiment and does not lead to as severe spin contamination but does allow for some relaxation in the CCSD(T) calculations.18,19,20,21 In addition, the explicitly correlated CCSD(T)-F12 30,31 method was also employed for geometry optimization using the cc-pVnZ-F12 (n = T, Q) basis sets.

32

The

CCSD(T) calculated electronic energies with n= D, T, and Q were extrapolated to the complete basis set (CBS) limit using a mixed Gaussian/exponential equation (1). E(n) = ECBS + A exp[−(n − 1)] + B exp[−(n − 1)2]


33

(1)


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The CCSD(T) electronic energies calculated with n = Q and 5 and for VTZ-F12 and VQZ-F12 were extrapolated to the CBS limit with equation (2) for each set. 34,35,36 E(lmax) = ECBS + B/lmax3

(2)

with an lmax = n. The composite thermochemistry approach of Feller-Peterson-Dixon18,19,20,21 was used to calculate the total atomization energies (ΣD0,0K, TAE) using equation (3). ΣD0,0K = ΔEZPE + ΔECBS + ΔECV +ΔESR + ΔESO

(3)

The core-valence (CV) correlation corrections for the 1s2 electrons on C and N and 2s22p6 electrons for S and P were calculated at the CCSD(T) level with the aug-cc-pwCVTZ basis sets. 37 Scalar relativistic (SR) corrections were calculated with the second-order Douglas-KrollHess Hamiltonian 38,39,40 and the all-electron aug-cc-pVTZ-DK basis set. 41 The spin orbit corrections (ΔESO) for the atoms were taken from the experiment. 42 (The spin-orbit corrections for the atoms are calculated as (ΣJ(J(2J + 1)·E(J)))/(ΣJJ(2J + 1)).) The spin orbit correction for N and P is zero. The ground state atomic spin-orbit corrections are ΔESO(C) = -0.09, ΔESO(O) = 0.22, and ΔESO(S) = -0.56 kcal/mol. By using the heats of formation at 0 K for the elements: 43 ΔHf,0K (C) = 169.98 ± 0.1 kcal/mol, ΔHf,0K (O) = 58.98 kcal/mol, ΔHf,0K (S) = 65.66 ± 0.06 kcal/mol, ΔHf,0K (N) = 112.53± 0.02 kcal/mol and ΔHf,0K (P) = 75.42 ± 0.24 kcal/mol, we can derive the heats of formation of the studied molecules. The heats of formation at 298 K can then be calculated using the approach described by Curtiss et al. 44 The CCSD(T) calculations were done with the MOLPRO program. 45,46 Results and Discussion Geometries Both singlet and triplet states were investigated, with the results for most of the triplets in the Supporting Information. The T1 diagnostics 47 for the singlets are about 0.02 for


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most structures except for the two high energy structures (Supporting Information) for the [C,N,O,P] isomers. The higher energy triplets show larger values of T1, but these will not impact the energy differences for the low energy structures. The lowest energy structure for [C,N,O,P] is a singlet and has a trivalent P with a P=O bond and a C≡N group (Table 1, Figure 1). The

Table 1. Relative Energies ΔH(0K) in kcal/mol and Geometry Parameters for PNCO Singlet Isomers at the CCSD(T)-F12b/VQZ Level.a Isomer

ΔH(0K)

r1

r2

r3

A1

0.0

1.164

1.843

1.472

1.3

1.186

1.724

1.464

TA4

13.0

1.171

1.200

A3

31.2

1.675

A4

37.5

A5

∠1 173.4

∠2 105.8

Dihedral ∠ 180.0

1.651

180.0

180.0

180.0

1.373

1.191

71.3

142.5

180.0

1.632

1.205

1.171

180.0

180.0

66.2

1.160

1.308

1.664

176.9

120.4

180.0

A6

81.4

1.554

1.381

1.224

171.6

115.7

180.0

A7

104.7

1.804

1.375

1.221

62.6

140.1

180.0

A8

120.8

1.179

1.356

1.640

172.5

118.7

180.0

A9c

125.5

1.765

1.709

1.474

62.9

100.3

24.6

A10

139.7

1.546

1.365

1.280

177.6

115.9

180.0

A2 b

a

166.2

107.7

180.0

Distances in Å. Bond Angles in degrees. b Triplet O=C=N-P. c VTZ=F12b.

cyano complex is the most stable structure with the isonitrile group orientation 1.3 kcal/mol higher in energy. The next highest energy isomer is the linear triplet (O=C=N-P) with the unpaired electrons localized on the P and it is 11.1 kcal/mol higher in energy than the ground state singlet O=P-C≡N. The bond distances in the triplet are consistent with C=O and C=N double bonds and a P-N single bond. The next higher energy isomer with a central C is a singlet and is much higher in energy, 31 kcal/mol above the lowest energy isomer. The CN bond 5 ACS Paragon Plus Environment


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distance is 0.02 Å longer in the isonitrile isomer than in the cyano isomer and the variation of the P=O bond distance is ~ 0.01 Å for the two isomers. The P-C bond is 0.12 Å longer than the P-N bond. The PCN of PNC angle is near linear and the OPN or OPC angle is strongly bent near 107°.

A2

A1

A5

A9

A4

A3

A6

A7

A8

A10

Figure 1. Isomers of [C,N,O,P]. Red = O, orange = P, grey = C, and blue = N.

The T1 diagnostics for the lowest energy singlets are about 0.02 to 0.03 with slightly larger values for the higher energy structures (Supporting Information) for the [C,N,S,P] isomers. The lowest energy isomer for [C,N,S,P] is clearly the singlet S=P-C≡N isomer with the singlet isonitrile isomer 6.4 kcal/mol higher in energy (Table 2, Figure 2). The same variations in the CN and P-C vs P-N bond distances is predicted for these compounds as found for O=P-C≡N


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and O=P-N≡C even though the P=S bond is much longer. Just as found for the [C,N,O,P] series, the next highest energy isomer is the linear triplet (S=C=N-P) with the unpaired electrons localized on the P and it is 19 kcal/mol higher in energy than the ground state singlet S=P-C≡N. In contrast to the [C,N,O,P] series, the next highest energy isomer is the compound P=S-C≡N, which is 36 kcal/mol higher in energy with the isomer with a central carbon 37 kcal/mol higher in energy. Thus the ability for S to increase its valency as compared to O enables the formation of the lower energy P=S-C≡N isomer. Table 2. Relative Energies ΔH(0K) in kcal/mol and Geometry Parameters for [C,N,S,P] Singlet Isomers at the CCSD(T)-F12b/VQZ Level.a

a

Isomer

ΔH(0K)

r1

r2

r3

S1

0.0

1.164

1.809

S2

6.4

1.185

TS5

18.6

S3

1.922

∠1

174.8

∠2

104.3

Dihedral ∠

1.716

1.907

169.7

107.0

180.0

1.206

1.578

1.646

180.0

180.0

180.0

36.0

1.163

1.753

1.954

178.4

109.6

-179.8

S4

37.0

1.660

1.375

1.605

71.1

142.1

180.0

S5

39.0

1.606

1.213

1.569

180.0

180.0

180.0

S6

52.2

1.183

1.770

1.897

174.1

113.6

-180.0

S7

54.2

1.576

1.297

1.602

174.0

124.3

180.0

S8

73.6

1.488

1.779

1.556

112.8

179.3

180.0

S9

81.8

1.795

1.376

1.614

63.2

139.2

180.0

S10

86.8

1.709

1.699

1.711

107.1

68.8

23.3

S11

97.0

1.619

1.798

1.641

90.7

94.9

0.0

S12

105.0

1.767

1.799

1.507

74.3

163.1

180.0

S13

115.7

2.179

1.558

1.731

76.4

104.8

0.0

S14

128.3

1.831

1.712

1.473

69.7

145.3

180.0

Distances in Å, Angles in Degrees)


180.0


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Vibrational frequencies The vibrational frequencies for OPCN have been reported to be 2165 cm-1 for the CN stretch and 1385 cm-1 for the PO stretch,13 although a reanalysis of the spectra suggest that the value is actually 2105 cm-1.15 The corresponding calculated values are 2196 cm-1 and 1237 cm-1 (Table 3). The calculated harmonic value for the CN stretch is higher than the 2165 cm-1 experimental value as expected, given that the anharmonic correction for the CN

Figure 2. Isomers of [C,N,S,P]. Yellow = S, orange = P, grey = C, and blue = N.

stretch 48 in HCN is 15 cm-1 giving a total anharmonic correction of 30 cm-1. Similarly, the experimental harmonic frequency 49 of CN diatomic is 2068.59 cm-1 and the VQZ-F12b value is 2071.9 cm-1. The experimental value49 of ωexe is only 13.087 cm-1, so we would expect an error 8 ACS Paragon Plus Environment

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on the order of 25-30 cm-1 as compared to experiment. Thus, our calculated value would be in good agreement with the initial experimental value of 2165 cm-1 but not with the revised value of 2105 cm-1. The calculated value for the P=O stretch is also expected to be within 13 cm-1 of experiment at this level as the experimental value49 for ωexe for PO is only 6.56 cm-1. The calculated value is smaller than experiment by ~ 150 cm-1 consistent with the prior DFT and MP2 results.15 Thus, the experimental assignment of the PO frequency as being due to the OPCN molecule is incorrect and the revised experimental value15 for the CN stretch is also not consistent with the calculated value. In addition, the isonitrile frequencies show an even worse fit so these cannot account for the observed transitions.

Table 3. Harmonic Vibrational Frequencies (υ, cm-1) and Rotational Constants (A, B, C, MHz) for [C,N,O,P] Singlet Isomers at the CCSD(T)-F12b/VQZ Level molecul e

ω1

ω2

ω3

ω4

ω5

ω6

A

B

C

A1

2196

1237

590

421

197

160

29171

3807

3367

2054

1272

637

415

132

121

32971

4225

3744

TA4

2352

1451

607

560 π

A3

1895

1040

840

624

24140

4315

5254

A4

2345

1437

636

534c π

A5

2301

1080

746

518

446

192

46019

3448

3207

A6

1524

1395

761

569

245

184

76320

2950

2840

A7

1560

1060

756

709

482

431

21940

4520

5693

A8

2103

820

748

435

228

145

47847

3562

3315

A9b

1164

880

666

623

523

480

21599

7060

5557

A10

1532

1183

730

522

309

192

62050

3092

2945

A2 a

a

46 π 510

434 90d π

2656

CCSD(T)/aug-cc-pV(T+D)Z. b F12b-T. c Average of 625/443 cm-1. d Average of 116/64 cm-1



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In a similar manner, it has previously been shown16 that the transitions assigned to SPCN cannot be correct. We can confirm these results as our calculated harmonic frequencies should be for the PS and PC stretches should be within 15 cm-1 of the experimental transitions and that for the CN stretch within 30 cm-1 of these transitions. Our calculated values (Table 4) of 691 cm-1 for the PS stretch and 591 cm-1 for the PC stretch clearly differ from the experimental values14 of 743, and 622 cm-1 by more than 15 cm-1, and our calculated value of 2191 cm-1 for the CN stretch differs by 40 cm-1 from the experimental value of 2151 cm-1. The 2151 cm-1 band has been re-evaluated to be at 2156 cm-1 and was assigned to the CN stretch in NCCN based on spectral profiles and the position of the cyanogen band at 2158 cm-1.16 Our values are more Table 4. Harmonic Vibrational Frequencies (υ, cm-1) and Rotational Constants (A, B, C, MHz) for [C,N,S,P] Singlet Isomers at the CCSD(T)-F12b/VQZ Level Molecule

ω1

ω2

ω3

ω4

ω5

ω6

A

B

C

S1

2191

696

591

403

244

142

14301

2325

2777

S2

2054

720

625

354

156

130

16829

2472

2898

S3

2195

620

564

403

291

143

15414

2308

2714

S4

1410

977

739

544

388

310

23052

2781

2482

1967

1070

537

428 π

150 π

2037

1044

508

449 π

110 π

S6

2042

700

429

296

124

99

18692

2363

2705

S7

1456

1018

691

465

313

142

51262

1885

1818

S8

1400

1189

530

366

174

124

31485

1967

2098

S9

1258

926

677

566

372

297

20921

3001

2625

S10

946

833

776

605

460

309

19359

4195

3546

S11

984

902

669

504

458

205

12587

5330

3744

S12

1285

791

666

288

207

150

19205

2944

3478

S13

967

907

753

469

228

74

12259

3641

5178

S14

1303

812

642

541

244

226

20430

3786

3194

S5 TS5

a

a

b

c

1615

CCSD(T)/aug-cc-pV(T+D)Z. b Average of 506/351 cm-1. c Average of 162/137 cm-1 10 ACS Paragon Plus Environment

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accurate than the prior DFT or MP2 values16 and do confirm the prior semi-quantitative results. Thus, the experimental transitions cannot be assigned to SPCN as noted previously. Although the calculated PS (720 cm-1) and PN (625 cm-1) stretches in SPNC are similar to the experimental values,14 the calculated CN stretch of 2050 cm-1 is clearly not consistent with experiment for this higher energy isomer. Thermodynamic Properties The components of the total atomization energies and the heats of formation are given in Table 5. The atomization energies with the DTQ extrapolation are ~ 1.0

Table 5. Components for FPD calculations and heats of formation for the 2 lowest energy isomers in kcal/mol Property

O=P-CN

O=P-NC

S=P-CN

S=P-NC

ΔEelec CBS-DTQ

413.39

411.96

370.81

364.16

ΔEelec CBS-Q5

414.39

412.82

371.76

364.95

ΔEelec F12b- TQ

414.33

412.86

371.45

364.73

ΔEZPE

-6.80

-6.55

-6.04

-5.71

ΔESR

-0.56

-0.60

-0.26

-0.29

ΔEso

-0.31

-0.31

-0.65

-0.65

ΔECV

1.99

1.81

1.99

1.80

ΣD0 DTQ

407.72

406.31

365.85

359.31

ΣD0 Q5

408.72

407.17

366.81

360.11

ΣD0 F12b-TQ

408.66

407.21

366.49

359.88

ΔHf,0K DTQ

9.2

10.6

4.2

10.7

ΔHf,298K DTQ

9.0

10.4

4.0

10.6

ΔHf,0K Q5

8.2

9.7

3.2

9.9

ΔHf,298K Q5

8.0

9.6

3.1

9.8

ΔHf,0K F12b-TQ

8.2

9.7

3.6

10.2

ΔHf,298K F12b-TQ

8.1

9.5

3.4

10.0



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kcal/mol less than the extrapolated Q5 and TQ-F12 extrapolated values. The latter two values agree within 0.3 kcal/mol. The core-valence corrections are positive and about 2 kcal/mol and the scalar relativistic corrections are all less than -0.6 kcal/mol. We expected the lowest energy bond in the lowest energy isomers to be the P-C or P-N single bonds as nominally the PO and PS bonds are double bonds and the CN bonds are triple bonds in the typical Lewis dot picture of O=P-C≡N and S=P-C≡N where the lone pair on P is not shown. We have previously calculated the heats of formation of CN, PO, and PS at the FPD level21 giving respective values of 104.0 ± 0.3, −7.1 ± 0.4, and 38.1 ± 0.3 kcal/mol. These give respective BDEs for CN, PO, and PS of 125, 195, and 103 kcal/mol. We can write that the atomization energy is the sum of the 3 individual BDEs and we take the BDEs for the CN and PO or PS fragments to be those of the diatomics. In fact this is equivalent to taking the differences in the appropriate heats of formation for the reaction OPCN → PO + CN, as an example. The P-C (or P-N) BDE is then given by the following expression: BDE(P-C or P-N) = ΣD0 – BDE(CN) – BDE(PO or PS) The P-C BDEs are then 89 kcal/mol for O=P-CN and 139 kcal/mol for S=P-CN and P-N BDEs of 87 kcal/mol for O=P-NC and 132 kcal/mol for S=P-NC. What is of course interesting is that the P-C or PN bond is the weakest bond in molecules with O but that in the molecules with S, the P-C or P-N bonds are stronger than the P=S bond. In fact, there is an additional stabilization in the SP-CN or SP-NC bonds. It is also of interest to examine the stability of the lowest lying triplets which are linear 3

O=C=N-P and 3S=C=N-P. The C-N BDE for 3O=C=N=P with the products NP (ΔHf = 43.0 ±

0.4 kcal/mol) and CO (ΔHf = 26.4 ± 0.3 kcal/mol) is 48 kcal/mol.21 In contrast, the P-N BDE in 3

O=C=N-P can be estimated to be 88 kcal/mol using the best reported value 50 of ΔHf(NCO) =


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31.4 ± 0.5 kcal/mol. For 3S=C=N-P, the corresponding C-N BDE is 90 kcal/mol using a value of ΔHf(CS) = 67.0 ± 0.3 kcal/mol. The corresponding P-N BDE in 3S=C=N-P is 70 kcal/mol using a value of ΔHf,298 (SCN) = 16.8 kcal/mol calculated using the FPD approach as described here. The difference in the C-N BDEs in the triplets is due to the difference in the stability of the CO and CS fragments and the difference in the P-N BDEs for the triplets is due in part to the different stabilities of OCN and SCN. Electronic Structure Populations and orbital occupancies were determined using NBO6 51,52 for the natural bond orbital (NBO) 53,54,55,56 population analysis. The NBO calculations were carried out using the B3LYP hybrid functional 57,58 with the DZVP2 basis set 59 for Si, N, S, and P. The density functional theory calculations were done using Gaussian 09. 60 The NPA charges of the molecules are given in Table 6. The PO bond is significantly more polar than the P-S bond. The P has more positive charge in the isonitrile than in the cyano compounds. There is a much larger charge separation on the C and N in the isonitrile than in the cyano compounds. There is a significant ionic interaction in the isonitrile compounds between the N and P due to the large differences in the charges.

Table 6. NPA charges (e) at the B3LYP/DZVP2 Molecule

O

O=P-CN O=P-NC

S

P

C

N

-0.86

1.25

-0.09

-0.29

-0.89

1.44

0.37

-0.92

S=P-CN

-0.24

0.58

-0.05

-0.29

S=P-NC

-0.28

0.80

0.37

-0.88

In all of the compounds, there are two lone pairs on O and S, one of which is depleted by up to 0.13 e. There is always a lone pair on P and there is a lone pair on N for the cyano 13 ACS Paragon Plus Environment


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compounds and a lone pair on C in the isocyano compounds. There is always a C≡N triple bond and there is always a P=O or P=S double bond. There is a P-C or P-N single bond depending on the structure. Of interest is that there is always a P-C* or P-N* component to the σ bonding. The P-C* component is about 0.10 e and the P-N* component is 0.12 to 0.14 e. Comparison to nitrogen counterparts and astrochemical significance Carbon, nitrogen, oxygen, and sulfur are some of the most cosmically abundant elements outside of hydrogen and helium. Their reactions with phosphorus are important in the formation of pre-biotic species. The recent tentative detection of NCCP near IRC +10216 61 opens the possibility for other tetratomic or larger phosphorus-bearing unsaturated molecules in similar stellar environments. Unlike [C,N,O,P] and [C,N,S,P], the set of isoelectronic [N,N,C,O] isomers have been thoroughly studied both experimentally62,63,64,65,66,67,68 and theoretically. 69,70,71,72,73,74,75,76 Of interest to astrochemistry, it has been suggested that [N,N,C,O] isomers are detectable in the Kuiper belt found in the outer solar system beyond Neptune.65 In contrast to the current results found for [C,N,O,P], the global minimum has been found to be the cyc-NNC=O structure. The NCNO, NNCO, and NNCO structures are predicted to be 14.2, 7.4, and 7.4 kcal/mol higher in energy than the global minimum at the QCISD(T)/6-311+G*//MP2/6-31+G* level.75 Experimentally, the NCNO, CNNO, and NCON structures have been observed in Ar matrices.68 The triplet NNCO molecule was identified in neutralization-reionization mass spectrometry experiments.62 The cyc-NNC=O molecule was observed from pyrolysis synthesis67 and from electron irradiated CO2-N2 ices in an ultra-high vacuum at 10K.65 The dissociation of NC-PO to CN + PO is endothermic by 88.8 kcal/mol at 298 K. An alternative dissociation channel involving significant rearrangement of the lowest energy structure is to CO + PN; in contrast, this channel is endothermic by only 8.5 kcal/mol. For the


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corresponding NC-NO molecule, we used the same FPD approach (CBS values using the aug-ccpVnZ with n = D, T, Q) with the geometry optimized at the MP2/aug-cc-pVTZ level and with frequencies at this level (See Supporting Information). The calculated heat of formation of NCNO is 24.6 kcal/mol at this level. The dissociation into CN + NO using comparable calculated values21 for CN and NO (ΔHf(298) = 21.9 kcal/mol) is endothermic by ~101.3 kcal/mol,75 ~ 12 kcal/mol more endothermic than what we find for formation of CN and PO. However, the dissociation of NC-NO into CO (ΔHf(298) = -26.4 kcal/mol) and N2 is exothermic by -51.0 kcal/mol kcal/mol. (We note that there is a significant error75 in the previous report of the instability of this molecule. The energy of decomposition of cyc-NNC=O into N2 and CO is exothermic by -36.5 kcal/mol rather than the near 100 kcal/mol previously reported.75) Thus the difference in the stability (bond energies) of N2 and PN of 80 kcal/mol has a profound difference on the stability of corresponding compounds NC-PO and NC-NO with respect to the lowest energy dissociation channel. Conclusions The CNPO and NCPO molecules together with their sulfur analogues are the simplest molecules possessing phosphorus along with one member from each of these group 14, 15, and 16 elements and represent the lowest energy stable structures in their respective isomer sets. The formation of CNPO or NCPO in stellar environments could be possible through the addition of the previously detected CN and PO diatomics and the formation of the sulfur analogues would be possible by a similar addition reaction if the PS diatomic is identified in extraterrestrial environments in the future. Our results provide reliable energetics for these species as well as good estimates of the geometries for rotational transition studies as well as good estimates of the harmonic vibrational frequencies, which should aid in their possible detection. In addition, our



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high level calculations of the frequencies clearly show that the previously assigned transitions13,14 are incorrect as discussed previously15,16 on the basis of lower level calculations. Acknowledgment. The work at The University of Alabama is supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) (geosciences program). DAD also thanks the Robert Ramsay Chair Fund of The University of Alabama for support. Supporting Information. Complete reference for references 45 and 60.

Total energies,

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