Since the Sq 2-molecule has a high symmetry, the principal axes of the symmetric parts of each shielding tensor are all precisely aligned with a molecular symmetry axis. The principal values of all four C shielding tensors are the same and are calculated to be σ XX = -68.6 ppm, σ Y Y = -97.4 ppm, σ ZZ = +74.7 ppm. For each C site, the X principal axis is parallel to the line joining that site with the centre of the square, the Z principal axis is perpendicular to the plane of the square, and the Y axis is perpendicular to both X and Y. Figure depicts the calculated shielded tensors as blue and orange ovaloid surfaces 107,108 superposed on the Sq 2-molecular structure. The energy-minimized structure of the SqH -molecule has low symmetry, as shown in figure . Assume that the proton is attached to oxygen O1, which is bound to carbon C1. Define an axis system such that the x-axis is parallel to the line joining C1 to the centre of the square, the z-axis is perpendicular to the square, and the y-axis is perpendicular to the other two axes. The shielding tensors for the four carbon sites, expressed in this axis system, are as follows: (A1)