CHAPTER SIX Naphthalene
153 6-4 The Second Singlet State (S2) of Naphthalene
The second singlet state, S2 (as 1B1u), shows higher shielding values of both
nuclear (σiso0 Å) and σ1 Åiso than in the ground state of naphthalene, see tables (6- 3) and (6-1), respectively. However, the increase for the σiso1 Å values is larger than that for σiso0 Å. For the S2 state, the increase is around one-third more than
of S0 state value. Interestingly, all the carbon nuclei in S2 naphthalene show
a significant shielding increase but C8a nucleus shows the largest increase in both σiso0 Å and σ1 Åiso parameters. As a result, the C4a-C8a bond has different shielding profile in comparison to the previous two states.
The |Δσiso(C−C)0 Å | and |Δσiso(C−C)1 Å | values for the C1-C8a and C1-C2 bonds are (3.87 / 12.53) and (2.79 / 2.42) (ppm); respectively. This shows that the maximum variation is observed for the points at 1 Å above the ends of the C1- C8a bond. Also, the high σiso values in table (6-3) can be considered as a sign
of the strong shielding and aromaticity of this electronic state.
Table 6-3: CASSCF(10,10)/6-311+G(d) isotropic shielding data (σiso) (in ppm) for S2
naphthalene for symmetry unique carbon nuclei, and for points placed 1Å above these nuclei.
Nuclear σiso (ppm)
σiso 1 Å above nuclei (ppm)
C1 83.68 20.77
C2 80.89 23.19
C8a 79.81 33.30
Comparing the 2D results from the previous two sections to those for the S2
state shows a substantial increase in shielding, see figures (6-(17 to 21)). A glance at figure (6-17) shows immediately that the isotropic shielding of the S2 state is very much the opposite of that for the S1 state. Both rings are well-
shielded, with intensive shielding clouds around the bonds. However, the shielding clouds of the C-C bonds show significant shifts towards the inside region of the rings (see map (a)). Also, the most intensive circular shielding clouds inside each ring are found at heights of 0.50-1.00 Å, see maps (c-e). This indicates strong shielding activity at those heights. Surprisingly, the figure shows the highly shielded areas prefer to be much more contracted inside the rings than what has been found for the ground state (see figure (6-1)). σiso at
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1 Å is dominated by σiso(zz) because of the high similarity between the σiso and
σiso(zz) plots in figures (6-17 (e)) and (6-18).
Figure 6-17: Contour maps of the isotropic shieldings (σiso) (in ppm) obtained at the
CASSCF(10,10)/6-311+G(d) level of theory for S2 naphthalene. (a-e): for
grids respectively placed at 0.0, 0.25, 0.50, 0.75 and 1.00 Å vertical heights above the molecular plane.
It can be argued that when electron delocalisation becomes more pronounced, the ring current and the resulting shielding clouds prefer shorter circular paths inside the rings. These strong ring shielding clouds prepare an
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adequate magnetic environment which works as an attractive factor causing the C-C bonds shielding clouds to be shifted to the inner regions of the rings. The shielding at different heights above the ring centre shows similar behaviour. This behaviour is hard to notice in figures (6-17,19,20) but the curve in figure (6-22) is easier to follow. When comparing the shielding profile of this curve to that for the ground state in figure (6-6), we notice important differences. For the ground state, the maximum isotropic shielding is 10.16 (ppm) at 0.77 Å. The value for the S2 state is 42.52 (ppm) at 0.51 Å. This
means the highest shielding associated with electron delocalisation moves much closer to the molecular plane when the overall shielding of the ring increase (i.e. with an increase in the aromaticity).
Figure 6-18: Contribution of the vertical-component of the isotropic shielding (σiso(zz)) (in
ppm) of S2 naphthalene, obtained at the CASSCF(10,10)/6-311+G(d) level of
theory for a grid placed vertically at 1.00 Å above the molecular plane.
As it can be seen in figure (6-19), the shielded region has a considerable vertical bulk. It reaches 3.5 Å in height, which is much higher than that in the ground state (see figure (6-3)). Both figures show similarities in shielding behaviour. For both states, deshielded regions are found at the left and the right sides of the molecule.
Another important observation is that the shielding of the C4a-C8a bond is much higher than those around other C-C bonds. As a consequence, the deshielded-spheres of this bond behave differently from the general trend for the other C-C bonds, see figures (6-17 (a,b)) and (6-21 (d)).
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Figure 6-19: XZ cross-section isotropic shielding map (in ppm) of S2 naphthalene,
obtained at the CASSCF(10,10)/6-311+G(d) level of theory for a grid placed vertically and bisecting the molecular plane at its X and Z axes.
Figure 6-20: CASSCF(10,10)/6-311+G(d) isotropic shielding contour maps (in ppm) of S2
naphthalene. The figures result from 2D ghost-atom grids located perpendicularly to and crossing the midpoints of symmetry unique C-C bonds. Note that for all maps above, the right side is directed towards the inner part of the ring, which includes the ring centre. Also, the C-C bond in each map is located exactly at the (0,0) X,Y coordinates.
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The deshielded-spheres of the nuclei in the C4a-C8a bond show distortions in their shapes, they are not centred on the carbon nuclei, and incorporate significant variations in their levels of deshielding. As it can be seen in figure (6-21 (d)), the lowest levels of deshielding in these spheres are observed inside the bond. The most deshielded parts are located outside the bond. Further details can be seen in curves (a1-a4) in figure (6-23). The deshielding parts of the C4a-C8a bond are the shortest ones in comparison to other C-C bonds, while most of the curve is strongly shielded. Also, the lowest deshielding values in the figure are within the deshielded-spheres of this bond. All these features are caused by the strong shielding of the bond. This is a feature example of competition between shielding and deshielding activities.
The arrangement the C-C bonds according to shielding levels is as follows: C4a-C8a > C2-C3 > C1-C2 > C1-C8a, see figures (6-20,21,23). Although this order differs from that in the ground state, however, the bond with weakest shielding is C1-C8a in both states.
A further observation associated with figure (6-23). The shielding at 1 Å above the C-C bonds is shown by the red curves (b) which follows the same order as that mentioned above for the corresponding bonds, curves (a). Additionally, the shielding above the bonds shows that the minimum shielding values are located above the nuclei, except that the above C8a nucleus in the C1-C8a bond, see curve (b2). The reason for this exception can be explained by looking at figure (6-17 (e)) and curve (6-21 (b)). The two maps show that the shielding clouds of the two rings are linked over the C4a-C8a bond, making the spaces at 1 Å above these nuclei much more shielded. There are no such shielding effects over the C1 nucleus or any other carbon nuclei. Due to the very high isotropic shielding of S2 naphthalene, the +16 (ppm) blue
isosurface, does not show much details of the overall 3D shielding behaviour. Therefore, another value, +44 (ppm), has also been selected for a second shielding isosurface, the green isosurface, see figure (6-24).
On the other hand, deshielding activity can be well-represented by the -16 (ppm) isosurface only due to no more deshielding features to be shown by a further value.
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Figure 6-21: CASSCF(10,10)/6-311+G(d) isotropic shielding contour maps (in ppm) for the symmetry unique C-C bonds of S2 naphthalene.In each map, the left and
right well-shielded dots represent the C nuclei of the correspondingly bond. Also, the midpoint of each bond corresponds to the (0,0) X,Y coordinates.
Figure 6-22: Variation in CASSCF(10,10)/6-311+G(d) isotropic shielding σiso (in ppm) of S2
naphthalene obtained from 71 ghost-atoms located perpendicularly from the ring centre to a height of 2 Å above it.
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Figure 6-23: Variation in CASSCF(10,10)/6-311+G(d) isotropic shielding (σiso) (in ppm) of
the symmetry unique C-C bonds of S2 naphthalene obtained from 71 ghost-
atoms: (a) positioned along C-C bonds, and (b) positioned 1Å vertically above the bonds.
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Figure 6-23: Continued.
As it can be seen from the shielding profiles in figure (6-24 (a,b)), the blue isosurface represents the common trend in the shielding of aromatic molecules. The shielding cloud surrounds the molecule and shows a considerable vertical bulk. The shieldings over the C-H bonds are also displayed by this isosurface. These findings indicate that S2 naphthalene is
strongly aromatic.
However, several details can only be understood by looking at the +44 (ppm) green isosurface in figure (6-24 (c,d)). This green isosurface illustrates the vertical and the horizontal shrinkage of the strongly shielded region towards the insides of the rings. When comparing the expansion tendencies of the green and the blue isosurfaces, it becomes obvious that the stronger shieldings are closer to the molecular plane. The displacements in the locations of the shielding clouds of the C-C bonds can also be seen via the green isosurface. The higher isotropic shielding of the C4a-C8a bond, which links the shielding clouds of the two rings, cannot be seen in the +16 (ppm) isosurface, but is well-represented by the +44 (ppm) green isosurface.
Finally, the results reported in this section, show clearly that the shielding in the S2 state is much stronger than in ground state, S0, naphthalene.
Consequently, the S2 state has much more pronounced aromatic features.
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Figure 6-24: Isotropic shielding (σiso) isosurfaces of S2 naphthalene obtained from
CASSCF(10,10)/6-311+G(d) calculations. The blue isosurface corresponds to the +16 (ppm) shielding value. The green isosurface corresponds to the +44 (ppm) shielding value. Isotropic deshieldings are represented by the red isosurface which corresponds to -16 (ppm). (a,c) show the front-view; (b,d), show two side-views; (e) shows the cross-section of the three isosurfaces.
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