Structural characterization of monomers for non-linear optical polymers. X-ray analysis and molecular mechanics calculations of 2-(4-methacryloxy-3-methoxyphenyl)-1,1-dicyanoethylene, 2-(3-methacryloxy-4-methoxyphenyl)-1,1-dicyanoethylene, and 2-(2-methacryloxy-3-methoxyphenyl)-1,1-dicyanoethylene