Proposal for sets of se NMR chemical shifts in planar and perpendicular orientations of aryl group and the applications.

The orientational effect of p-YC(6)H(4) (Ar) on delta(Se) is elucidated for ArSeR, based on experimental and theoretical investigations. Sets of delta(Se) are proposed for pl and pd employing 9-(arylselanyl)anthracenes (1) and 1-(arylselanyl)anthraquinones (2), respectively, where Se-C(R) in ArSeR is on the Ar plane in pl and perpendicular to the plane in pd. Absolute magnetic shielding tensors of Se (sigma(Se)) are calculated for ArSeR (R = H, Me, and Ph), assuming pl and pd, with the DFT-GIAO method. Observed characters are well reproduced by the total shielding tensors (sigma(t)(Se)). The paramagnetic terms (sigma(P)(Se)) are governed by sigma(P)(Se)(xx) + sigma(P)(Se)(yy), where the direction of n(P)(Se) is set to the z-axis. The mechanisms of the orientational effect are established both for pl and pd. Sets of delta(Se: 1) and delta(Se: 2) act as the standards for pl and pd, respectively, when delta(Se) of ArSeR are analyzed based on the orientational effect.

INTRODUCTION

77 Se NMR spectroscopy is one of powerful tools to study selenium compounds [1-20], containing bioactive materials [21-24]. 77 Se NMR chemical shifts (δ(Se)) are sharply sensitive to the structural changes in selenium compounds. Therefore, they are widely applied to determine the structures [6-20] and to follow up the reactions of selenium compounds [1-10]. The δ(Se) values have been analyzed variously. The substituent effect is employed when the effect of the electronic conditions around Se on δ(Se) is examined in p-YC perturbed by Y, for example [6-20]. Some empirical rules and/or classifications between structures and δ(Se) are proposed [6-20], however, it is not so easy to predict δ(Se) from the structures with substantial accuracy. Some important rules would be behind the observed values. Plain rules, founded on the theoretical background, are necessary to analyze the structures of selenium compounds based on δ(Se) and also to understand the origin of δ(Se) [25].

We have pointed out the importance of the orientational effect on δ(Se) of p-YC, for the better understanding of δ(Se) of ArSeR in a uniform manner [19, 20, 25]. To establish the orientational effect, we present two series of δ(Se) for p-YC whose structures (conformers) are fixed to planar (pl) and perpendicular (pd) conformers for all Y examined, under the conditions [26, 27]. (The nonplanar and nonperpendicular conformer (np) is also important in some cases, such as the CC conformer in 1,8-(MeZ) (Z = S and Se) [28-33].) (The importance of relative conformations in the substituent effects between substituents and probe sites is pointed out.) The Se−CR bond in ArSeR is on the Ar plane in pl and perpendicular to the plane in pd. 9-(Arylselanyl)anthracenes (p-YC: 1) and 1-(arylselanyl)anthraquinones (p-YC: 2) are the candidates for pl and pd, respectively: Y in 1 and 2 are H (a), NMe (b), OMe (c), Me (d), F (e), Cl (f), Br (g), COOEt (h), CN (i), and NO (j) (see Chart 1). Conformers of the 9-anthracenyl (9-Atc) and 1-anthraquinonyl (1-Atq) groups in 1 and 2 are represented by the type A (A), type B (B), and type C (C) notation, which is proposed for 1-(arylselanyl)naphthalenes (p-YC: 3) [14–16, 19, 20, 26]. The structure of 1 is A for 9-Atc and pl for Ar, which is denoted by 1 (A: pl). That of 2 is B for the 1-Atq and pd for Ar (2 (B: pd)). The series of δ(Se) in 1 (δ(Se: 1)) and δ(Se: 2) must be typical for pl and pd, respectively.

Chart 1

Recently, the reliability of the calculated absolute magnetic shielding tensors (σ) is much improved [34-39] and the calculated tensors for Se nuclei (σ(Se)) are demonstrated to be useful in usual selenium compounds [28-33]. As shown in (1), the total absolute magnetic shielding tensor (σ) is decomposed into diamagnetic (σ) and paramagnetic (σ) contributions [40, 41]. σ contributes predominantly to σ in the structural change of selenium compounds. Magnetic shielding tensors consist of three components, as exemplified by σ in (2) as the following:

Quantum chemical (QC) calculations are performed on ArSeH (4), ArSeMe (5), and ArSePh (6) to understand the orientational effect based on the theoretical background (see Chart 1). The conformations are fixed to pl and pd in the calculations. The gauge-independent atomic orbital (GIAO) method [42-46] is applied to evaluate σ(Se) at the DFT (B3LYP) level. Mechanisms of the orientational effect are explored for pl and pd based on the magnetic perturbation theory on the molecules.

After the establishment of the orientational effect of aryl group in p-YC, together with the mechanism, δ(Se) of some aryl selenides are plotted versus δ(Se: 1) and/or δ(Se: 2). The treatment shows how δ(Se) of aryl selenides are interpreted based on the orientational effect. And it is demonstrated that the sets of δ(Se: 1) and δ(Se: 2) give a reliable guideline to analyze the structures of p-YC based on δ(Se).

RESULTS

The structures of all members of 1 and 2 are predicted to be 1 (A: pl) and 2 (B: pd), respectively [25]. The results are supported by the X-ray crystallographic analysis carried out for 1 and 2, containing 1c and 2a and the QC calculations for 1a and 2a, together with the spectroscopic measurements, although not shown. Scheme 1 illustrates 1 (A: pl) and 2 (B: pd), together with some conformers of 3.

Scheme 1

Structures of 1 and 2, together with those of 3.

Table 1 shows δ(Se: 1) and δ(Se: 2), measured in chloroform-d solutions (0.050 M) at 213 K, 297 K, and 333 K. δ(Se: 1a) and δ(Se: 2a) are given from MeSeMe and δ(Se: 1) and δ(Se: 2) are from 1a and 2a, respectively, (δ(Se)SCS). To examine the temperature dependence in 1, δ(Se: 1)SCS at 297 K (δ(Se: 1)SCS, 297 K) and δ(Se: 1)SCS, 333 K are plotted versus δ(Se: 1)SCS, 213 K. Table 2 collects the correlations, where the correlation constants (a and b) and the correlation coefficients (r) are defined in the footnote of Table 2 (entries 1 and 2). δ(Se: 2)SCS, 297 K and δ(Se: 2)SCS, 333 K are similarly plotted versus δ(Se: 2)SCS, 213 K. Table 2 also contains the correlations (entries 3 and 4). The a values for 1 are smaller than those for 2. The results show that the temperature dependence in 1 is larger than that of 2, although both correlations are excellent (r > 0.999). The results show that 2 (B: pd) are thermally very stable and other conformers are substantially negligible in the solution for all Y examined. 1 (A: pl) must also be predominant in solutions, although 1 (A: pl) would not be thermally so stable, relative to the case of 2 (B: pd).

Table 1

Observed δ(Se)SCS of 1 and 2 and calculated σ rel(Se)SCS for 4–6 in pl and pd (.

Compd	T	NMe2	OMe	Me	H	F	Cl	Br	CO2R (c)	CN	NO2
	[K]	(b)	(c)	(d)	(a)	(e)	(f)	(g)	(h)	(i)	(j)
1	213	−22.7	−12.7	−6.3	0.0 (245.3)	−3.3	1.9	2.4	17.4	27.7	32.7
1	297	−21.0	−12.2	−6.6	0.0 (249.0)	−3.6	1.5	1.6	16.2	26.2	30.4
1	333	−21.3	−12.7	−6.8	0.0 (250.6)	−3.9	1.0	1.2	15.2	24.8	29.0
2	213	−20.6	−15.5	−9.2	0.0 (511.4)	−10.5	−7.1	−6.4	0.1	8.5	2.7
2	297	−19.6	−15.0	−9.0	0.0 (512.3)	−10.2	−7.1	−6.4	0.0	8.2	2.5
2	333	−19.5	−15.0	−9.1	0.0 (512.5)	−10.3	−7.2	−6.7	−0.3	7.9	2.2
4 (pl)	—	−36.4	−18.0	−8.2	0.0 (87.0)	−1.6	1.7	−1.8	14.3	29.8	33.7
4 (pd)	—	−35.9	−23.0	−15.6	0.0 (41.3)	−11.8	−9.1	−8.7	1.0	16.8	10.0
5 (pl)	—	−23.9	−8.2	−8.0	0.0 (169.7)	2.1	4.7	7.2	24.6	29.7	43.8
5 (pd)	—	−34.9	−21.2	−16.7	0.0 (219.1)	−14.1	−11.8	−12.6	3.0	13.4	6.6
6 (pl)	—	−20.5	−9.0	−3.7	0.0 (398.8)	1.1	1.9	2.3	13.1	20.2	28.6
6 (pd)	—	−34.2	−25.8	−14.6	0.0 (398.8)	−15.2	−13.3	−12.6	−3.4	7.0	0.5

(a) δ(Se)SCS are given for 1 and 2, together with δ(Se) for 1 a and 2 a in parenthesis, measured in chloroform-d.

(b) σ rel(Se)SCS are given for 4−6, together with σ rel(Se) for 4a−6a in parenthesis, calculated according to (3), where σ(Se) of 4−6 in pl and pd are given in Tables 3–5, respectively, and σ(Se: MeSeMe) = 1650.4 ppm.

(c) R = Et for 1 and 2 and R = Me for 4−6.

Table 2

Correlations of δ(Se)SCS for 1 and 2 and σ(Se) for 4−6, together with δ(Se)SCS for 5−9 .

Entry	Correlation	a	b	r	n (b)
1	δ(Se: 1)SCS, 297 K vs δ(Se: 1)SCS, 213 K	0.940	−0.3	1.000	10
2	δ(Se: 1)SCS, 333 K vs δ(Se: 1)SCS, 213 K	0.916	−0.8	1.000	10
3	δ(Se: 2)SCS, 297 K vs δ(Se: 2)SCS, 213 K	0.957	−0.1	1.000	10
4	δ(Se: 2)SCS, 333 K vs δ(Se: 2)SCS, 213 K	0.946	−0.3	1.000	10
5	δ(Se: 1)SCS, 213 K vs σ rel(Se: 4 (pl))SCS	0.823	2.6	0.986	10
6	δ(Se: 1)SCS, 213 K vs σ rel(Se: 5 (pl))SCS	0.845	−2.1	0.990	10
7	δ(Se: 1)SCS, 213 K vs σ rel(Se: 6 (pl))SCS	1.218	−0.4	0.991	10
8	δ(Se: 2)SCS, 213 K vs σ rel(Se: 4 (pd))SCS	0.562	−1.5	0.990	10
9	δ(Se: 2)SCS, 213 K vs σ rel(Se: 5 (pd))SCS	0.599	−0.5	0.988	10
10	δ(Se: 2)SCS, 213 K vs σ rel(Se: 6 (pd))SCS	0.691	1.9	0.990	10
11	σp(Se) vs (σp(Se)xx + σp(Se)yy) in 4 (pl)	0.339	−547.8	0.982	10
12	σp(Se) vs (σp(Se)xx + σp(Se)yy) in 5 (pl)	0.367	−461.8	0.999	10
13	σp(Se) vs (σp(Se)xx + σp(Se)yy) in 6 (pl)	0.350	−546.7	0.990	10
14	σp(Se) vs (σp(Se)xx + σp(Se)yy) in 4 (pd)	0.309	−547.0	0.998	10
15	σp(Se) vs (σp(Se)xx + σp(Se)yy) in 5 (pd)	0.345	−517.4	0.994	10
16	σp(Se) vs (σp(Se)xx + σp(Se)yy) in 6 (pd)	0.335	−598.5	0.998	10
17	δ(Se: 5)SCS (c) vs δ(Se: 1)SCS, 213 K	0.997	1.0	0.997	8
18	δ(Se: 5)SCS (d) vs δ(Se: 1)SCS, 213 K	0.952	0.1	0.999	7
19	δ(Se: 7)SCS vs δ(Se: 2)SCS, 213 K	0.909	1.3	0.995	10
20	δ(Se: 6)SCS vs δ(Se: 1)SCS, 213 K	0.804	−3.3	0.991	7
21	δ(Se: 8)SCS vs δ(Se: 1)SCS, 213 K	0.691	−1.7	0.981	9
22	δ(Se: 9)c SCS vs δ(Se: 1)SCS, 213 K	0.870	−1.3	0.999	7

(a)The constants (a, b, r) are defined by y = ax + b (r: correlation coefficient).

(b)The number of data used in the correlation. (c)Reference [19] at neat. (d)Reference [11] in CDCl.

Scheme 2 shows the axes and some orbitals of 4–6, together with SeH. While the x-axis of SeH is in the bisected direction of ∠HSeH, the Se−H and Se−C bonds of MeSeH are almost on the x- and y-axes, respectively, although not shown. Axes of 4–6 are close to those in MeSeH in most cases. Since ∠CSeX (X = H or C) in 4–6 are about 95°, 98°, and 101°, respectively, the Se−C and Se−H bonds deviate inevitably from the axes to some extent. Axes are rather similar to those of SeH for 4 (pl) with Y = Br and COOMe and 5 (pl) with Y = Me and CN.

Scheme 2

Axes and some orbitals of 4–6, together with those of SeH.

Structures of 4–6 in pl and pd are optimized employing the 6-311+G(3df) basis sets for Se and the 6-311+G(3d,2p) basis sets for other nuclei of the Gaussian 03 program [47]. Calculations are performed at the density functional theory (DFT) level of the Becke three parameter hybrid functionals with the Lee-Yang-Parr correlation functional (B3LYP). Absolute magnetic shielding tensors of Se (σ(Se)) are calculated based on the DFT-GIAO method [42-46], applying on the optimized structures with the method. Tables 3–5 collect σ(Se), σ(Se), σ(Se), and the components of σ(Se), σ(Se), σ(Se), and σ(Se) for 4–6 bearing various substituents Y in pl and pd, respectively.

Table 3

Calculated absolute shielding tensors (σ(Se)) of 4, containing various Y .

Y	σd(Se)	σp(Se)xx	σp(Se)yy	σp(Se)zz	σp(Se)	σt(Se)
4 (pl)
H	2999.5	−1571.7	−1042.3	−1694.2	−1436.1	1563.4
NMe2	3006.4	−1676.1	−862.4	−1681.4	−1406.7	1599.8
OMe	3004.7	−1823.5	−757.0	−1689.5	−1423.3	1581.4
Me	3002.4	−1760.2	−848.1	−1684.2	−1430.8	1571.6
F	3001.4	−1800.4	−833.2	−1675.7	−1436.4	1565.0
Cl	3003.8	−1777.8	−868.7	−1680.0	−1442.2	1561.7
Br	3008.7	−1883.4	−745.3	−1701.6	−1443.4	1565.2
COOMe	3010.0	−1469.6	−1197.4	−1715.7	−1460.9	1549.1
CN	3002.1	−1829.1	−889.8	−1686.5	−1468.5	1533.6
NO 2	3004.9	−1836.6	−905.8	−1683.2	−1475.2	1529.7
4 (pd)
H	3001.9	−1870.9	−869.9	−1437.6	−1392.8	1609.1
NMe2	3004.1	−1782.2	−842.4	−1452.8	−1359.1	1645.0
OMe	3005.4	−1805.2	−871.3	−1443.6	−1373.4	1632.1
Me	3002.2	−1821.7	−871.0	−1439.8	−1377.5	1624.7
F	3000.8	−1829.8	−866.2	−1443.7	−1379.9	1620.9
Cl	3000.8	−1834.5	−870.2	−1442.8	−1382.5	1618.2
Br	3000.5	−1835.5	−870.5	−1442.1	−1382.7	1617.8
COOMe	3004.2	−1872.6	−879.2	−1436.5	−1396.1	1608.1
CN	2999.9	−1901.0	−881.6	−1440.1	−1407.6	1592.3
NO 2	3000.7	−1877.7	−884.4	−1442.8	−1401.6	1599.1

(a) Structures are optimized with the 6-311+G(3df) basis sets for Se and 6-311+G(3d,2p) basis sets for other nuclei at the DFT (B3LYP) level, assuming pl and pd for each of Y [47]. σ(Se) are calculated based on the DFT-GIAO method with the same methods.

Table 5

Calculated absolute shielding tensors (σ(Se)) of 6, containing various Y .

Y	σd(Se)	σp(Se)xx	σp(Se)yy	σp(Se)zz	σp(Se)	σt(Se)
6 (pl)
H	2995.1	−1527.4	−1887.5	−1815.6	−1743.5	1251.6
NMe2	2997.7	−1462.8	−1902.3	−1811.6	−1725.5	1272.1
OMe	2995.5	−1504.1	−1887.4	−1813.2	−1734.9	1260.6
Me	2995.6	−1517.7	−1888.8	−1814.2	−1740.3	1255.3
F	2994.5	−1544.4	−1879.4	−1808.1	−1743.9	1250.5
Cl	2994.1	−1550.2	−1873.8	−1809.2	−1744.4	1249.7
Br	2996.5	−1553.1	−1871.1	−1817.4	−1747.2	1249.3
COOMe	2997.2	−1574.5	−1871.7	−1830.0	−1758.7	1238.5
CN	2994.8	−1605.6	−1869.2	−1815.6	−1763.5	1231.4
NO2	2994.4	−1630.8	−1867.8	−1815.7	−1771.4	1223.0
6 (pd)
H	2995.1	−1887.5	−1527.4	−1815.6	−1743.5	1251.6
NMe2	2998.3	−1787.3	−1531.6	−1818.6	−1712.5	1285.8
OMe	3002.2	−2044.1	−1313.9	−1816.4	−1724.8	1277.4
Me	2996.4	−1843.1	−1532.7	−1814.8	−1730.2	1266.2
F	2994.8	−1851.2	−1517.7	−1815.0	−1728.0	1266.8
Cl	2995.1	−1859.5	−1519.1	−1812.1	−1730.2	1264.9
Br	2997.2	−1871.4	−1514.8	−1812.8	−1733.0	1264.2
COOMe	3003.2	−2085.4	−1341.9	−1817.2	−1748.2	1255.1
CN	2998.5	−2132.1	−1310.6	−1818.8	−1753.8	1244.6
NO2	2995.5	−1914.6	−1502.6	−1816.1	−1744.4	1251.1

(a)Structures are optimized with the 6-311+G(3df) basis sets for Se and 6-311+G(3d,2p) basis sets for other nuclei at the DFT (B3LYP) level, assuming pl and pd for each of Y [47]. σ(Se) are calculated based on the DFT-GIAO method with the same methods.

Relative shielding constants of A (σ rel(Se: A)) are calculated for 4–6 according to (3), using σ(Se: MeSeMe) (= 1650.4 ppm). σ rel(Se: A)SCS are calculated similarly. Table 1 also contains σ rel(Se: A) of 4a–6a and σ rel(Se: A)SCS for 4–6,

Table 6 shows σ(Se)SCS of p-YC (7) [13], p-YC (8) [8], and bis[8-(arylselanyl)naphthyl] 1,1′-diselenides (9) [15, 16], together with 5 [11, 19] and 6 [15, 16] (see Chart 2). The values are plotted versus δ(Se: 1)SCS and/or δ(Se: 2)SCS to explain the δ(Se) based on the orientational effect of the aryl groups.

Table 6

Observed δ(Se)SCS reported for 5−9.

Compd	NMe2	OMe	Me	H	F	Cl	Br	CO2R (a)	CN	NO2
	(b)	(c)	(d)	(a)	(e)	(f)	(g)	(h)	(i)	(j)
5 (b)	−20.8	−10.4	−7.2	0.0 (207.8)	—	2.5	2.8	20.1	—	33.4
5 (c)	—	−12.5	−5.9	0.0 (202.0)	−2.0	1.6	—	16.1	—	31.4
6 (b)	—	−15.5	−8.6	0.0 (423.6)	—	−1.7	−1.3	9.7	—	22.7
7 (d)	−18.6	−12.6	−7.1	0.0 (641.5)	−7.1	−4.5	−4.1	0.8	8.9	4.2
8 (e)	—	−12.0	−7.8	0.0 (320.8)	−2.5	0.2	0.9	8.6	21.0	18.0
9 (f)	—	−9.8	−6.6	0.0 (434.3)	—	−2.7	−1.9	8.1	—	19.6

(a) R = Me for 5 and R = Et for 6−9. (b)Reference [19]. (c)Reference [11] at neat. (d)Reference [13]. (e)Reference [8]. (f)Reference [15, 16].

Chart 2

DISCUSSION

Characters in δ(Se: 1) and δ(Se: 2)

The structures of all members of 1 and 2 are confirmed to be 1 (A: pl) and (B: pd), respectively, (see Scheme 1) [25]. The nature of δ(Se: 1) must be the results of 1 (A: pl), where n(Se) is parallel to the π(C-p). Characteristic points in δ(Se: 1)SCS are summarized as follows.

Large upfield shifts (−23 ppm to −6 ppm) are observed for Y = NMe, OMe, and Me and large downfield shifts (17 ppm to 33 ppm) are for Y = COOEt, CN, and NO, relative to Y = H.

Moderate upfield shift (−3 ppm) is observed for Y = F.

Small downfield shifts (2 ppm) are for Y = Cl and Br: the three points corresponding to Y = H, Cl, and Br are found very close with each other.

The characters of δ(Se: 2)SCS are very different from those of δ(Se: 1)SCS. The characteristics must be the reflection of 2 (B: pd), where n(Se) is perpendicular to π(C). Characteristic points of δ(Se: 2)SCS are as follows.

Large upfield shifts (−21 to −6 ppm) are observed for Y = NMe, OMe, Me, F, Cl, and Br, relative to Y = H.

Downfield shifts (3 ppm to 9 ppm) are brought by Y = CN and NO 2, where the magnitude by Y = CN is larger than that by NO 2.

δ(Se: 2)SCS brought by Y = COOEt is negligible.

While δ(Se: 1)SCS is in a range of −23 < δ(Se)SCS < 33 ppm, δ(Se: 2)SCS is −21 < δ(Se)SCS < 9 ppm. Y of both donors and acceptors operate well on δ(Se: 1)SCS , whereas only Y of donors do well on δ(Se: 2)SCS.

δ(Se: 2)SCS are plotted versus those of δ(Se: 1)SCS. Figure 1 shows the results. Indeed, it emphasizes the difference in the characters between δ(Se: 1)SCS and δ(Se: 2)SCS, but most of δ(Se: 2)SCS seem to correlate well with δ(Se: 1)SCS, as shown by a dotted line (a = 0.58). Two points corresponding to Y = H and NO deviate upside and downside from the line, respectively. Namely, points for 2 with Y of non-H are more downside (upfield) than expected from δ(Se: 1a)SCS and δ(Se: 2a)SCS, especially for δ(Se: 2j)SCS.

Figure 1

Plot of δ(Se: versus δ(Se: .

Why are such peculiar behaviors observed in 1 and 2, caused by the orientational effect of the aryl group? The mechanism is elucidated based on the QC calculations performed on 4–6, assuming pl and pd for each.

Observed δ(Se) versus calculated σ(Se)

The δ(Se)SCS values of 1 and 2 are plotted versus σ rel(Se)SCS of 4 (pl)–6 (pl) and 4 (pd)–6 (pd), respectively, (Table 1). Good correlations are obtained as shown in Table 2 (entries 5–10). The r values become larger in an order of 4(pl) < 5(pl) ≦ 6(pl) for 1 and in an order of 5(pd) < 4(pd) ≈ 6(pd) for 2. Namely, observed δ(Se: 1)SCS and δ(Se: 2)SCS are reproduced by σ rel(Se: 6 (pl))SCS and σ rel(Se: 6 (pd))SCS, respectively, in most successfully. Figure 2 exhibits the plots for (a) 1 versus 6 (pl) and (b) 2 versus 6 (pd). The correlations are given in Table 2 (entries 7 and 10). The results demonstrate that the characters of δ(Se)SCS observed in 1 originate from the planar structure and those in 2 from the characteristic structure, where Se−CAtq in p-YC is perpendicular to the p-YC plane.

Figure 2

Plots of (a) δ(Se: 1)SCS, 213 K versus σ rel(Se: 6 (pl))SCS and (b) δ(Se: 2)SCS, 213 K versus σ rel(Se: 6 (pd))SCS.

How does such orientational effect arise from the structures? How does the electronic property of Y affect on δ(Se) of 1 and 2? σ(Se) of 4–6 are analyzed next.

Orientational effect in 4a–6a

σ(Se) of 4–6 shown in Tables 3–5 are examined. σ(Se) and σ(Se) of 4a (pd) are evaluated to be larger (more upfield) than those of 4a (pl) by 43 ppm and 46 ppm, respectively, which correspond to the orientational effect caused by Ph in 4a. The inverse orientational effect is predicted for 5a. σ(Se) and σ(Se) of 5a (pd) are smaller than those of 5a (pl) by 41 ppm and 49 ppm, respectively. While σ(Se) and σ(Se) of 5a (pl) are smaller than those of 4a (pl) by 90 ppm and 83 ppm, respectively, the values of 5a (pd) are smaller than those of 4a (pd) by 174 ppm and 178 ppm, respectively. The differences are −84 ppm and −95 ppm, respectively, which also correspond to the differences in the orientational effect of the Ph group between 5a and 4a, respectively. The more effective contribution to downfield shifts by the Se−CMe bond in 5a (pd), relative to 5a (pl), must be responsible for the results. The orientational effect cannot be discussed for 6a of the Cs symmetry with Y = H.

What mechanism is operating in the Y dependence? σ(Se) of 4–6 in pl and pd are analyzed next.

Y dependence in 4–6

To get an image in the behavior of σ(Se), σ(Se), and σ(Se) of 4–6, the values are plotted versus σ(Se). Figure 3 shows the plots for 4 (pd) and 6 (pl). The correlations in 4 (pd) are linear and both σ(Se) and σ(Se) increase along with σ(Se). The plot for 5 (pd) is similar to that for 4 (pd), although not shown. In the case of 6 (pl), the correlations are linear but the slope for σ(Se) is inverse to that for σ(Se). The plots of σ(Se) and σ(Se) do not give smooth lines for 4 (pl), 5 (pl), and 6 (pd). However, the slopes for σ(Se) are very smooth and the magnitudes are very close to 1.0 for all cases in 4–6.

Figure 3

Plots of σ(Se) (●), σ(Se) (■), and σ(Se) (Δ) versus σ(Se): (a) for 4 (pd) and (b) for 6 (pl).

To clarify the behavior of σ(Se) in 4–6, σ(Se) are plotted versus (σ(Se) + σ(Se)). Excellent to good correlations are obtained in all cases as collected in Table 2 (entries 11–16). Figure 4 exhibits the plot of σ(Se) for 6 (pd), for example. The correlation constants (a) are 0.31–0.37, which are very close to one third. The results exhibit that (σ(Se)+σ(Se)) determines σ(Se) of 4–6 effectively (cf: (2)). The observations led us to establish the mechanism of Y dependence in 4–6.

Figure 4

Plot of σ(Se) versus σ(Se) + σ(Se) for 6 (pd).

Mechanism of Y dependence

The mechanism of Y dependence in 4–6 is elucidated by exemplifying 4. As shown in Scheme 2, the main interaction between Se and Y in 4 (pl) is the 4p(Se)-π(C)-p(Y) type, which modifies the contributions of 4p(Se) in π(SeC) and π*(SeC). Since (σ(Se) + σ(Se)) controls σ(Se) of 4 (pl) effectively, admixtures between 4p(Se) in modified π(SeC) and π*(SeCY) with 4p(Se) and 4px(Se) in σ(C) and σ*(C) must originate the Y dependence mainly when a magnetic field is applied. Since σ contains the ^L operator, σ arises from admixtures between atomic p and p orbitals of N in various molecular orbitals. When a magnetic field is applied on a selenium compound, mixings of unoccupied molecular orbitals (MO's; ψ ) into occupied orbital MO's (ψ ) will occur. Such admixtures generate σ if ψ and ψ contain p and p of N, for example. σ and σ are also understood similarly. Consequently, Y of both donors and acceptors are effective for the Y dependence in 4 (pl). Scheme 3(a) shows the mechanism for pl.

Scheme 3

Mechanisms of Y dependence. Outline allows exhibit the effect of p(Y) on 4p(Se) and double allows show the main admixtures to originate δ(Se): (a) in pl and (b) in pd.

In the case of 4 (pd), 4p(Se) remains in n(Se) in the almost pure form. The σ(C)-π(C)-p(Y) interaction occurs instead, which modifies the contributions of 4p and 4p in σ(C) and σ*(C) (see Scheme 2). (σ(Se) + σ(Se)) determines effectively σ(Se) of 4 (pd). Therefore, Y dependence of 4 (pd) originates mainly from admixtures between 4p(Se) in n(Se) and 4p(Se) and 4p(Se) in modified σ*(C) since n(Se) of 4p(Se) is filled with electrons. Consequently, Y dependence in 4 (pd) must be more sensitive to Y of donors, which is a striking contrast to the case of 4 (pl). Scheme 3(b) summarizes the mechanism for pd.

The mechanisms proposed for 4 (pl) and 4 (pd) must be applicable to 5 and 6. The expectations are just observed in δ(Se: 1)SCS and δ(Se: 2)SCS.

Applications of δ(Se: 1) and δ(Se: 2) as the standards

Odom made a lot of effort to explain δ(Se) of 7 based on the electronic effect of Y [13]. However, the attempt was not successful: δ(Se: 7) were not correlated well with δ(Se: 5). How are δ(Se) of p-YC interpreted based on the orientational effect? Our explanation for the relationship between δ(Se) of p-YC and the structures is as follows.

Figure 5 shows the plot of δ(Se: 5)SCS measured in CDCl [19] versus δ(Se: 1)SCS, 213 K and the correlation is given in Table 2 (entry 17: r = 0.997). The correlation coefficient is excellent when δ(Se: 5)SCS measured in neat is plotted versus δ(Se: 1)SCS, 213 K (entry 18 in Table 2: r = 0.999). These observations must be the results of the Se−CMe bond in 5 being on the p-YC plane in solutions for all Y examined, under the conditions. On the other hand, δ(Se: 7)SCS do not correlate with δ(Se: 1)SCS, 213 K. Instead, they correlate well with δ(Se: 2)SCS, 213 K (entry 19 in Table 2: r = 0.995). Figure 6 shows the plot. The results are rationally explained by assuming that the Se−CO bond in 7 is perpendicular to the p-YC plane in solutions for all Y examined, under the conditions.

Figure 5

Plot of δ(Se: 5)SCS versus δ(Se: 1)SCS, 213 k.

Figure 6

Plot of δ(Se: 7)SCS versus δ(Se: 2)SCS, 213 k.

δ(Se)SCS of 6 [19] and 8 [8] are similarly plotted versus δ(Se: 1)SCS, 213 K. They give good correlations, although the r values become poorer relative to that for 5 (entries 20 and 21 in Table 2). The reason would be the equilibrium of pl with pd for some Y in 6 and 8, may be Y of donors.

δ(Se)SCS of 9 are also plotted versus δ(Se: 1)SCS, 213 K. The correlations are excellent (entry 22 in Table 2: r = 0.999). It is worthwhile to comment that the energy lowering effect by Se 4c–6e in 9 fixes the conformation 9 (pl, pl) for both p-YC groups in solutions for all Y examined, under the conditions [50].

It is demonstrated that sets of δ(Se: 1) and δ(Se: 2) proposed in this work can be the standards for pl and pd, respectively, when δ(Se) of aryl selenides are analyzed based on the orientational effect.

CONCLUSION

The orientational effect is empirically established by the Y dependence on δ(Se: 1) and δ(Se: 2). The Y dependence observed in 1 and 2 is demonstrated by σ(Se) calculated for 4–6 with the DFT-GIAO method. While σ(Se) of 4a (pl) is predicted to be more negative than that of 4a (pd) by 46 ppm, σ(Se) of 5a (pl) is evaluated to be larger than that of 5a (pd) by 49 ppm, which corresponds to the orientational effect by the Ph group in 4a and 5a, respectively. Excellent to good correlations are obtained in the plots of σ(Se) versus (σ(Se)+σ(Se)) for 4–6 in pl and pd. It is demonstrated that (σ(Se) + σ(Se)) effectively controls σ(Se) of 4–6 in pl and pd.

The mechanisms of the Y dependence are proposed based on the magnetic perturbation theory. The main interaction in pl is the n(Se)-π(C)-p(Y) conjugation. Y dependence in pl occurs through admixtures of 4p(Se) in modified π(SeC) and π*(SeC) with 4p(Se) and 4p(Se) in σ(CSeX) and σ*(CSeX) (X = H or C). The main interaction in pd is the σ(CSeX)-π(C)-p(Y) type, which modifies both σ(C) and σ*(C). The Y dependence in pd mainly originates from admixtures of 4p(Se) in n(Se) with 4p(Se) and 4p(Se) in modified σ*(CSeX) since n(Se) of 4p(Se) is filled with electrons. Therefore, Y of both donors and acceptors are effective in pl, whereas Y of donors are more effective in pd. The expectations are just observed in 1 and 2. Sets of δ(Se: 1) and δ(Se: 2) can be used as the standards for pl and pd, respectively, when δ(Se) of aryl selenides are analyzed.

The effect of R in ArSeR is also important, which is in progress. The results will be discussed elsewhere, together with the applications of the method.

EXPERIMENTAL

NMR spectra were recorded at 25°C on a JEOL JNM-AL 300 spectrometer (1 H, 300 MHz; 13 C, 75.45 MHz; 77 Se, 57.25 MHz). The 1 H, 13 C, and 77 Se chemical shifts are given in parts per million relative to those of Me, internal CDCl in the solvent, and external MeSeMe, respectively.

Preparation of compounds

1a–1j were prepared by the reactions of anthracenylgrignard reagents with arylselanylbromides and/or aromatic diazonium salts with anthracenylselenolates as the case of 3 [14]. 2a–2j were prepared by the reactions of 8-chloroanthraquione and arylselenolates with CuI as described earlier [51]. Elementary analyses for the compounds were satisfactory to those calculated within ±0.3% accuracy. 1 H, 13 C, and 77 Se NMR chemical shifts of the compounds rationalize the structures.

MO calculations

Quantum chemical (QC) calculations were performed using a Silent-SCC T2 (Itanium2) computer with the 6-311+G(3df) basis sets for Se and 6-311+G(3d,2p) for other nuclei of the Gaussian 03 program [47]. Calculations are performed on 4–6 in pl and pd at the density functional theory (DFT) level of the Becke three parameter hybrid functionals combined with the Lee-Yang-Parr correlation functional (B3LYP). Absolute magnetic shielding tensors of Se nuclei (σ(Se)) are calculated based on the gauge-independent atomic orbital (GIAO) method, applying on the optimized structures with the same method.

Structures of 1a–3a in various conformers are also optimized, containing frequency analysis, with the B3LYP/6-311+G(d,p) method.

Table 4

Calculated absolute shielding tensors (σ(Se)) of 5, containing various Y .

Y	σd(Se)	σp(Se)xx	σp(Se)yy	σp(Se)zz	σp(Se)	σt(Se)
5 (pl)
H	3006.5	−1893.4	−999.0	−1684.9	−1525.8	1480.7
NMe2	3007.7	−1645.4	−1194.5	−1669.5	−1503.1	1504.6
OMe	3007.4	−1741.5	−1136.8	−1677.1	−1518.4	1488.9
Me	3008.0	−1815.2	−1064.7	−1678.0	−1519.3	1488.7
F	3006.2	−1911.7	−990.8	−1680.6	−1527.7	1478.6
Cl	3006.7	−1639.8	−1269.8	−1682.4	−1530.7	1476.0
Br	3008.1	−1768.8	−1156.2	−1679.0	−1534.7	1473.5
COOMe	3009.6	−1840.5	−1132.8	−1687.1	−1553.5	1456.1
CN	3006.6	−1601.6	−1377.0	−1688.1	−1555.6	1451.0
NO2	3007.0	−1800.0	−1220.1	−1690.0	−1570.1	1436.9
5 (pd)
H	2998.0	−1956.8	−1086.4	−1656.9	−1566.7	1431.3
NMe2	3003.5	−1889.2	−1062.0	−1660.9	−1537.3	1466.2
OMe	3004.1	−1938.6	−1059.6	−1656.6	−1551.6	1452.5
Me	2999.8	−1908.0	−1090.1	−1657.2	−1551.8	1448.0
F	2998.1	−1916.6	−1077.7	−1663.9	−1552.8	1445.4
Cl	2999.3	−1925.8	−1078.6	−1664.3	−1556.2	1443.1
Br	3001.0	−1930.0	−1077.7	−1663.5	−1557.1	1443.9
COOMe	3006.4	−2017.8	−1057.8	−1658.7	−1578.1	1428.3
CN	2998.0	−1995.5	−1076.6	−1668.2	−1580.1	1417.9
NO2	2999.5	−1977.4	−1075.9	−1671.0	−1574.7	1424.7

(a)Structures are optimized with the 6-311+G(3df) basis sets for Se and 6-311+G(3d,2p) basis sets for other nuclei at the DFT (B3LYP) level, assuming pl and pd for each of Y [47]. σ(Se) are calculated based on the DFT-GIAO method with the same methods.